Add proxy model test that fails in Django 1.5
Fix Django 1.5 support, tests pass again.

The reason polymorphic broke was because it couldn't find some managers
anymore in the inheritance tree. Django 1.5 removes these and replaces
them with an `AbstractManagerDescriptor`. This patch restores those objects
Merge pull request #31 from vdboor/django15

Fix Django 1.5 (proxy models broke polymorphism)