title: Working with Django Model Permission raises DoesNotExist error
body: <p>I have some problems with django guardian. I have a news model defined </p>
  
  <pre><code>class News(models.Model):
      title = models.CharField(_('Title'), max_length=255)
      slug = models.SlugField(_('Slug'), unique_for_date='pub_date',)
  
      objects = models.Manager()
      featured = FeaturedNewsManager()
  
      link = models.URLField(_('Link'), blank=True, null=True,)
  
      class Meta:
          permissions = (('view_news', _('view news')))
  </code></pre>
  
  <p>Then I try to assign the view_news permission to one of my users, and I get the following error:</p>
  
  <pre><code>&gt;&gt;&gt; from guardian.shortcuts import assign_perm
  &gt;&gt;&gt; g = Group.objects.latest('pk')
  &gt;&gt;&gt; n = News.objects.get(pk=4)
  &gt;&gt;&gt; assign_perm( 'news.view_news', g, n)
  Traceback (most recent call last):
    File "&lt;input&gt;", line 1, in &lt;module&gt;
    File "/home/roberto/.virtualenvs/ve_news/local/lib/python2.7/site-packages/guardian/shortcuts.py", line 93, in assign_perm
      return model.objects.assign_perm(perm, group, obj)
    File "/home/roberto/.virtualenvs/ve_news/local/lib/python2.7/site-packages/guardian/managers.py", line 90, in assign_perm
      permission = Permission.objects.get(content_type=ctype, codename=perm)
    File "/home/roberto/.virtualenvs/ve_news/local/lib/python2.7/site-packages/django/db/models/manager.py", line 143, in get
      return self.get_query_set().get(*args, **kwargs)
    File "/home/roberto/.virtualenvs/ve_news/local/lib/python2.7/site-packages/django/db/models/query.py", line 404, in get
      self.model._meta.object_name)
  DoesNotExist: Permission matching query does not exist.
  </code></pre>
  
  <p>I have tried migrating my app but that doesn't seem to solve my problem. Any help please?</p>
  
  <p>Thanks!</p>