title: Attribute Error 'module' object has no attribute 'DateField'
body: <p>I'm trying to extend an admin definition in Satchmo/Django and am getting the error "Attribute Error 'module' object has no attribute 'DateField'" trying to add a formfield_override to ProductOptions:</p>
  
  <pre><code>from django.contrib import admin
  from django.conf import settings
  from widgets import AdminImageWidget, AdminDateWidget
  
  from django.forms import models, ValidationError
  from django.utils.translation import ugettext_lazy as _
  from l10n.l10n_settings import get_l10n_setting
  from l10n.utils import moneyfmt
  from livesettings import config_value
  from product.models import *
  #from product.utils import import_validator, validate_attribute_value
  #from satchmo_utils.thumbnail.field import ImageWithThumbnailField
  #from satchmo_utils.thumbnail.widgets import AdminImageWithThumbnailWidget
  #from django.http import HttpResponseRedirect
  import re
  
  from product.admin import *
  
  
  class ProductImage_Inline(ProductImage_Inline):
      model = ProductImage
      extra = 0
  
      formfield_overrides = {
          ImageWithThumbnailField : {'widget' : AdminImageWidget},
      }
  
  class ProductOptions(ProductOptions):
  
      #list_display += ('slug', 'name', 'formatted_price', 'formatted_inventory', 'active','featured', 'get_subtypes')
      list_display = ('name', 'sku', get_categories, 'formatted_price', 'active','featured', 'get_subtypes')
  
      list_display_links = ('name',)
      list_filter = ('category', 'date_added','active','featured')
      actions = ('make_active', 'make_inactive', 'make_featured', 'make_unfeatured')
      fieldsets = (
          (None, {'fields': ('name', 'slug', 'sku')}), 
          (None, {'fields': ('site', 'category')}), 
          (None, {'fields': ('description', 'short_description', 'date_added','active', 'featured', 'items_in_stock','total_sold','ordering', 'shipclass')}), 
          (_('Meta Data'), {'fields': ('meta',), 'classes': ('collapse',)}),
          (_('Item Dimensions'), {'fields': (('length', 'length_units','width','width_units','height','height_units'),('weight','weight_units')), 'classes': ('collapse',)}),
          (_('Tax'), {'fields':('taxable', 'taxClass'), 'classes': ('collapse',)}),
          (_('Related Products'), {'fields':('related_items','also_purchased'),'classes':('collapse',)}))
  
      search_fields = ['slug', 'sku', 'name', 'category__name']
      inlines = [ProductAttribute_Inline, Price_Inline, ProductImage_Inline]
      if get_l10n_setting('show_admin_translations'):
          inlines.append(ProductTranslation_Inline)
      filter_horizontal = ('category',)
  
  #     formfield_overrides = {
  #       models.ImageField: {'widget': AdminImageWidget}}
  
      formfield_overrides = {
          models.DateField: {
              'widget': AdminDateWidget,
              'input_formats': settings.VALID_DATE_FORMATS,
              },
          }
  
      def formfield_for_dbfield(self, db_field, **kwargs):
          field = super(ProductOptions, self).formfield_for_dbfield(db_field, **kwargs)
          fieldname = db_field.name
          if fieldname in ("length_units", "width_units", "height_units"):
              field.initial = default_dimension_unit()
          elif fieldname == "weight_units":
              field.initial = default_weight_unit()
          return field
  
  admin.site.unregister(Product)
  admin.site.register(Product, ProductOptions)
  </code></pre>
  
  <p>Other types of fields that are in the model and should be available (CharField) generate the same error.</p>
  
  <p>Any ideas as to where I should look to fix this?</p>