title: Admin Models do not recognize inherited fields
body: <p>I am wondering if this is something specific to django.admin, django, or even python? But I don't really understand the point in having abstract super classes if I cannot access their fields :-). Have I done something wrong?</p>
  
  <p>Example:
  I get a FieldError with the following "Exception Value: Unknown field(s) (creation_date) specified for Module. Check fields/fieldsets/exclude attributes of class ModuleAdmin" if I use the admin interface to get the following model "Module":</p>
  
  <pre><code>class GeneralModel(models.Model):
      creation_date = models.DateTimeField('date of creation', auto_now_add=True)
      edited_date = models.DateTimeField('date of last modification', auto_now=True)
  
      class Meta:
          abstract = True
  
  
  class Module(GeneralModel):
      name = models.CharField(max_length=100)
      shortDescription = models.CharField("summary", max_length=100)
      description = models.CharField("description", max_length=1500)
      authors = models.ManyToManyField("Author", through="Authorship")
  
      def __unicode__(self):
          return self.name
  </code></pre>
  
  <p>With the following ModelAdmin Code:</p>
  
  <pre><code>class ModuleAdmin(admin.ModelAdmin):
      def formfield_for_dbfield(self, db_field, **kwargs):
          formfield = super(ModuleAdmin, self).formfield_for_dbfield(db_field, **kwargs)
          if db_field.name == 'description':
              formfield.widget = forms.Textarea(attrs=formfield.widget.attrs)
          return formfield
      fieldsets = [
          ("General", {"fields": ["name", "shortDescription"]}),
          ("Details", {"fields": ["description", "creation_date"], "classes": ["collapse"]})
      ]
  </code></pre>