Add ability to filter an InheritanceQuerySet by model.

This is based upon the feature in django-polymorphic, where you
can do:

    SuperClass.objects.instance_of(SubClass)

This will result in only objects of the subclass being fetched.

Note: this works with any queryset, keeping the filtering and
ordering applied there.

Is this something that would be considered? I'll write up some tests if that is the case.