Django 1.7

In django 1.7, app-loading was refactor and because `django-authority` uses `get_user_model`, an `AppRegistryNotReady: Models aren't loaded yet.` was thrown. 

A common fix to this error is to use `settings.AUTH_USER_MODEL`, specified in [docs](http://django.readthedocs.org/en/latest/ref/applications.html#applications-troubleshooting)