Proxy models support

Hi,

I've verified and tested the proxy model stuff that @floppya provided in PR #25. This pull request contains those commits, rebased on top of master (please merge after #33), plus a few fixes:

* I've excluded the admin log fixes, as they depend on a Django 1.6-dev patch. Without the patch, everything works fine in Django 1.5.
* Added proxy models to the example app.
* Avoid making extra queries, instead upcast the models directly (by assigning `__class__`).