Add ShopifyUserBackend so that the token can be stored against the User.
Rename ShopifyUserBackend => ShopUserBackend
Add an abstract ShopUser base class that other apps can extend for their auth user.
ShopUser => AbstractShopUser
UserOwnedMixin should inherit from models.Model in order to work properly.
Rename UserOwnedMixin to UserOwnedModel, and add a custom manager for UserOwnedModels.
Ensure __unicode__ method on AbstractShopUser does indeed return unicode.