Fixed authentication backend

A custom user model without a field called "username" can be used as
`AUTH_USER_MODEL`, the backend now returns `None` for a user model
without a "username" field allowing the next backend (if any) to
handle authentication. Previously it was raising: "FieldError: Cannot
resolve keyword 'username' into field."
Merge pull request #15 from tucarga/auth-backend-custom-user

Fixed authentication backend