title: Manager isn't accessible via `Model` instances
body: <p>I have polymorphic tagging model and I want to create tag_cloud for it,
  but when I wanna count related object to tags </p>
  
  <pre><code>tags = TaggedItem.objects.all()
  # Calculate tag, min and max counts.
  min_count = max_count = tags[0].object.objects.count()
  </code></pre>
  
  <p>i get:</p>
  
  <pre><code>Manager isn't accessible via Artcle instances
  </code></pre>
  
  <p>tagging.models.py</p>
  
  <pre><code>class Tag(models.Model):
      name = models.CharField(max_length=100)
      slug = models.SlugField(unique=True, max_length=100)
      #.....
  
  class TaggedItem(models.Model):
      tag = models.ForeignKey(Tag)
      content_type = models.ForeignKey(ContentType)
      object_id = models.PositiveIntegerField()
      object = generic.GenericForeignKey('content_type', 'object_id')
      #.....
  </code></pre>