Use Django's AUTH_USER_MODEL rather than the default User model

[borwick]

As referenced in the Django documentation: https://docs.djangoproject.com/en/dev/topics/auth/customizing/#auth-custom-user

Instead of referring to User directly, you should reference the user model using django.contrib.auth.get_user_model(). This method will return the currently active User model – the custom User model if one is specified, or User otherwise.

When you define a foreign key or many-to-many relations to the User model, you should specify the custom model using the AUTH_USER_MODEL setting. For example:

[lmorchard]

Hmm, looks like this fails tests under Django 1.4, which we still need to support. Will need to take a look at how to stay backwards compatible