title: Django admin-tools menu for Applications does not shown after I subclass SiteAdmin
body: <p>I'm using Django with django-admin-tools. After I subclass the SiteAdmin and used it to register my admin classes, the whole Application menu disapears.
  My modification on index and app_index is just adding a picture property from admin_model to model_dict</p>
  
  <pre><code>class CustomAdminSite(AdminSite):
      @never_cache
      def index(self, request, extra_context=None):
          """
          Displays the main admin index page, which lists all of the installed
          apps that have been registered in this site.
          """
          app_dict = {}
          user = request.user
          for model, model_admin in self._registry.items():
              app_label = model._meta.app_label
              has_module_perms = user.has_module_perms(app_label)
  
              if has_module_perms:
                  perms = model_admin.get_model_perms(request)
  
                  # Check whether user has any perm for this module.
                  # If so, add the module to the model_list.
                  if True in perms.values():
                      info = (app_label, model._meta.module_name)
                      model_dict = {
                          'name': capfirst(model._meta.verbose_name_plural),
                          'perms': perms,
                      }
                      if hasattr(model_admin, 'picture'):model_dict['picture'] = model_admin.picture
                      if perms.get('change', False):
                          try:
                              model_dict['admin_url'] = reverse('admin:%s_%s_changelist' % info, current_app=self.name)
                          except NoReverseMatch:
                              pass
                      if perms.get('add', False):
                          try:
                              model_dict['add_url'] = reverse('admin:%s_%s_add' % info, current_app=self.name)
                          except NoReverseMatch:
                              pass
                      if app_label in app_dict:
                          app_dict[app_label]['models'].append(model_dict)
                      else:
                          app_dict[app_label] = {
                              'name': app_label.title(),
                              'app_url': reverse('admin:app_list', kwargs={'app_label': app_label}, current_app=self.name),
                              'has_module_perms': has_module_perms,
                              'models': [model_dict],
                          }
  
          # Sort the apps alphabetically.
          app_list = list(six.itervalues(app_dict))
          app_list.sort(key=lambda x: x['name'])
  
          # Sort the models alphabetically within each app.
          for app in app_list:
              app['models'].sort(key=lambda x: x['name'])
  
          context = {
              'title': _('Site administration'),
              'app_list': app_list,
          }
          context.update(extra_context or {})
          return TemplateResponse(request, self.index_template or
                                  'admin/index.html', context,
                                  current_app=self.name)
  
  
      def app_index(self, request, app_label, extra_context=None):
          user = request.user
          has_module_perms = user.has_module_perms(app_label)
          app_dict = {}
          for model, model_admin in self._registry.items():
              if app_label == model._meta.app_label:
                  if has_module_perms:
                      perms = model_admin.get_model_perms(request)
  
                      # Check whether user has any perm for this module.
                      # If so, add the module to the model_list.
                      if True in perms.values():
                          info = (app_label, model._meta.module_name)
                          model_dict = {
                              'name': capfirst(model._meta.verbose_name_plural),
                              'perms': perms,
                          }
                          if hasattr(model_admin, 'picture'):model_dict['picture'] = model_admin.picture
                          if perms.get('change', False):
                              try:
                                  model_dict['admin_url'] = reverse('admin:%s_%s_changelist' % info, current_app=self.name)
                              except NoReverseMatch:
                                  pass
                          if perms.get('add', False):
                              try:
                                  model_dict['add_url'] = reverse('admin:%s_%s_add' % info, current_app=self.name)
                              except NoReverseMatch:
                                  pass
                          if app_dict:
                              app_dict['models'].append(model_dict),
                          else:
                              # First time around, now that we know there's
                              # something to display, add in the necessary meta
                              # information.
                              app_dict = {
                                  'name': app_label.title(),
                                  'app_url': '',
                                  'has_module_perms': has_module_perms,
                                  'models': [model_dict],
                              }
          if not app_dict:
              raise Http404('The requested admin page does not exist.')
          # Sort the models alphabetically within each app.
          app_dict['models'].sort(key=lambda x: x['name'])
          context = {
              'title': _('%s administration') % capfirst(app_label),
              'app_list': [app_dict],
          }
          context.update(extra_context or {})
  
          return TemplateResponse(request, self.app_index_template or [
              'admin/%s/app_index.html' % app_label,
              'admin/app_index.html'
          ], context, current_app=self.name)
  
  
  custom_site = CustomAdminSite(name='admin')
  custom_site.register(Group, GroupAdmin)
  custom_site.register(User, UserAdmin)
  custom_site.register(Site, SiteAdmin)
  </code></pre>
  
  <p>I have registered all my app models the same way as above, auth and sites menu are shown but none of my apps and their model_admin's</p>