title: Django ModelMultipleChoiceField object has no attribute to_field_name
body: <p>I'm trying to create a custom field for a ModelForm.  I'm extending from ModelMultipleChoiceField and then overriding render and render_options, however, I keep getting this exception when just trying to import my form:</p>
  
  <p><code>AttributeError: 'ModelMultipleChoiceField' object has no attribute 'to_field_name'</code></p>
  
  <p>I'm not sure what I'm missing. I've tried even adding a to_field_name attribute to my new class, but that doesn't help.  Here is my code:</p>
  
  <pre><code>class MultiSelect(ModelMultipleChoiceField):
  def __init__(self, queryset, cache_choices=False, required=True,
               widget=None, label=None, initial=None, help_text=None, *args, **kwargs):
      super(MultiSelect, self).__init__(queryset, cache_choices, required, widget,
              label, initial, help_text, *args, **kwargs)
  
  def render_options(self, name, choices, selected_choices):
      output = []
      i = 0
      for option_value, option_label in chain(self.choices, choices):
          checked_html = (option_value in selected_choices) and u' checked="checked"' or ''
          class_html = (i % 2 == 0) and u'even' or u'odd'
          output.append('&lt;li class="{0}"&gt;&lt;input type="checkbox" name="{1}" value="{2}"{3}/&gt;{4}&lt;/li&gt;'
                  .format(class_html, name, escape(option_value), checked_html, escape(option_label)))
          i += 1
  
  def render(self, name, value, attrs=None, choices=()):
      if value is None: value = []
      final_attrs = self.build_attrs(attrs, name=name)
      output = [u'&lt;ul class="multiSelect"&gt;']
      options = self.render_options(name, choices, value)
      if options:
          output.append(options)
      output.append('&lt;/ul&gt;')
      return mark_safe(u'\n'.join(output))
  
  
  class RoleForm(ModelForm):
      class Meta:
          model = Role
          exclude = ('user_id',)
          widgets = {
              'permissions': MultiSelect(queryset=Permission.objects.all())
          }
  </code></pre>
  
  <p>Whenever I simply do an <code>from myapp.forms import RoleForm</code>, I get the error above.</p>
  
  <p>Should I be adding something to my class that I'm missing?</p>