Django 1.7 support w/auth.User Migration fix

This addresses an issue that got created when an auto migration was performed with South when auth.User is in use. By default South will hardcode auth.User references when they should be dynamic. This pull request builds on the Django 1.7 request I submitted and additionally fixes the migration issue using this technique. 

http://kevindias.com/writing/django-custom-user-models-south-and-reusable-apps/

Closes #22