Proxy models

The bulk of the code is due to https://github.com/sdelements/django_polymorphic/tree/proxy_models which I've been using, albeit in an uncomplicated manner, successfully. This PR streamlines some of the more unrelated-to-proxy-models parts of that branch, brings it up to date with master and updates it for Django 1.5.

This takes advantage of new options available in ContentTypes for Django 1.5. For earlier Djangoes, a monkey-patch is applied.

Whether or not this is the code for it, I think proxy models would be a great addition.

OT: Thanks for maintaining this library!