title: How to translate a SlugField?
body: <p>I am trying to translate my model using django-modeltranslation and the SlugField is giving me a lot of trouble. I have successfully registered the fields for translation, as you can see in the db table description:</p>
  
  <pre><code>                                     Table "public.news_news"
      Column    |           Type           |                       Modifiers                        
  --------------+--------------------------+--------------------------------------------------------
   id           | integer                  | not null default nextval('news_news_id_seq'::regclass)
   title        | character varying(255)   | not null
   title_en     | character varying(255)   | 
   title_nb     | character varying(255)   | 
   slug         | character varying(50)    | not null
   slug_en      | character varying(50)    | 
   slug_nb      | character varying(50)    |
  </code></pre>
  
  <p>But when I try to add a new News item, I am getting the following error:</p>
  
  <pre><code>Traceback: File "/home/roberto/.virtualenvs/ve_news/local/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
    115.                         response = callback(request, *callback_args, **callback_kwargs) File "/home/roberto/.virtualenvs/ve_news/local/lib/python2.7/site-packages/django/contrib/admin/options.py" in wrapper
    372.                 return self.admin_site.admin_view(view)(*args, **kwargs) File "/home/roberto/.virtualenvs/ve_news/local/lib/python2.7/site-packages/django/utils/decorators.py" in _wrapped_view
    91.                     response = view_func(request, *args, **kwargs) File "/home/roberto/.virtualenvs/ve_news/local/lib/python2.7/site-packages/django/views/decorators/cache.py" in _wrapped_view_func
    89.         response = view_func(request, *args, **kwargs) File "/home/roberto/.virtualenvs/ve_news/local/lib/python2.7/site-packages/django/contrib/admin/sites.py" in inner
    202.             return view(request, *args, **kwargs) File "/home/roberto/.virtualenvs/ve_news/local/lib/python2.7/site-packages/django/utils/decorators.py" in _wrapper
    25.             return bound_func(*args, **kwargs) File "/home/roberto/.virtualenvs/ve_news/local/lib/python2.7/site-packages/django/utils/decorators.py" in _wrapped_view
    91.                     response = view_func(request, *args, **kwargs) File "/home/roberto/.virtualenvs/ve_news/local/lib/python2.7/site-packages/django/utils/decorators.py" in bound_func
    21.                 return func(self, *args2, **kwargs2) File "/home/roberto/.virtualenvs/ve_news/local/lib/python2.7/site-packages/django/db/transaction.py" in inner
    223.                 return func(*args, **kwargs) File "/home/roberto/.virtualenvs/ve_news/local/lib/python2.7/site-packages/django/contrib/admin/options.py" in add_view
    1036.             model_admin=self) File "/home/roberto/.virtualenvs/ve_news/local/lib/python2.7/site-packages/django/contrib/admin/helpers.py" in __init__
    35.         } for field_name, dependencies in prepopulated_fields.items()] File "/home/roberto/.virtualenvs/ve_news/local/lib/python2.7/site-packages/django/forms/forms.py" in __getitem__
    111.             raise KeyError('Key %r not found in Form' % name)
  
  Exception Type: KeyError at /en/admin/news/news/add/ Exception Value: u"Key 'slug' not found in Form"
  </code></pre>
  
  <p>My admin.py :</p>
  
  <pre><code>from cmsplugin_news.admin import NewsAdmin as OldNewsAdmin
  
  class NewsAdmin(OldNewsAdmin, TranslationAdmin,):
      fields = ('title', 'slug')
      form = NewNewsForm
  </code></pre>
  
  <p>models.py:</p>
  
  <pre><code>class News(models.Model):
      title = models.CharField(_('Title'), max_length=255)
      slug = models.SlugField(
          _('Slug'), unique_for_date='pub_date',
      )
  </code></pre>
  
  <p>and forms.py:</p>
  
  <pre><code>class NewNewsForm(forms.ModelForm):
      class Meta:
          model = News
          fields = ('title', 'slug', )
  </code></pre>
  
  <p>As well, this is my translation.py where I declare the fields that should be translated by django-modeltranslation. I suppose anyhow that the error is not here, as the fields are already translated in db:</p>
  
  <pre><code>class NewsTranslationOptions(TranslationOptions):
      fields = ('title', 'slug', 'excerpt', 'content')
  
  translator.register(News, NewsTranslationOptions)
  </code></pre>
  
  <p>Any help please? I am really stuck! Thanks!</p>