title: Display created/modified fields of django_extensions in admin interface
body: <p>I have a class based on <code>TimeStampedModel</code> from <code>django-extentions</code>:</p>
  
  <pre><code>from model_utils.models import TimeStampedModel
  
  
  class MyClass(TimeStampedModel):
      pass
  </code></pre>
  
  <p>By default in the admin interface the <code>created</code> and <code>modified</code> fields are not displayed in the edition page <code>my_app/myclass/id</code>.</p>
  
  <p>I tried this hack to force the display of the <code>created</code> and <code>modified</code> fields in the edit admin page for <code>MyClass</code>:</p>
  
  <pre><code>from django.contrib import admin
  
  from my_app.models import MyClass
  
  
  class MyClassAdmin(admin.ModelAdmin):
      fields = MyClass._meta.get_all_field_names()
  
  admin.site.register(MyClass, MyClassAdmin)
  </code></pre>
  
  <p>But this raised the following exception:</p>
  
  <pre><code>Exception Type:     FieldError
  Exception Value:    Unknown field(s) (modified, created) specified for MyClass. Check fields/fieldsets/exclude attributes of class MyClassAdmin.
  </code></pre>
  
  <p>Any idea how can I display the <code>created</code> and <code>modified</code> fields in the <code>MyClass</code> edition admin interface?</p>
  
  <p><strong>Note 1:</strong> <code>MyClass</code> is a model with a lot of fields including <code>ManyToMany</code> fields. I can display all the fields except the <code>created</code> and <code>modified</code> fields from the base class <code>TimeStampedModel</code>.</p>
  
  <p><strong>Note 2:</strong> The admin page in reference is the edition page of a row: <code>my_app/myclass/id</code></p>