We now use the related manager to access the model class rather than
importing at runtime which causes a circular reference.

Fixes issues reported in comments of #1127

Previously, the test suite ran with a custom user model for Django >=
1.5, but it didn't subclass Oscar's AbstractUser.  This commit adjusts
the custom user model to subclass Oscar's so we can test functionality
within the custom user class.

This does require a change to the way tests authenticate as we don't
actually use the username field.