title: Filtering only on Annotations in Django
body: <p>Taking the example from:
  <a href="http://docs.djangoproject.com/en/dev/topics/db/aggregation/#filter-and-exclude" rel="noreferrer">http://docs.djangoproject.com/en/dev/topics/db/aggregation/#filter-and-exclude</a></p>
  
  <pre><code>Publisher.objects.filter(book__rating__gt=3.0).annotate(num_books=Count('book'))
  </code></pre>
  
  <p>Is there anyway to have the filter only apply to the annotation, so it would return all publishers, with some having a num_books=0?</p>