Fix for app initialization with django 1.7

With the new app loading in Django 1.7 using `get_user_mode` doesn't work when running migrations because things haven't loaded properly. The solution is to use `settings.AUTH_USER_MODEL` instead.

Doc:
https://docs.djangoproject.com/en/1.7/topics/auth/customizing/#django.contrib.auth.get_user_model
Doc Commit:
https://github.com/django/django/pull/2416/files