Update testing for Django 1.5

* Emailbackend: Updated to use REQUIRED_FIELDS
* Demo and sandbox site now use compat.AUTH_USER_MODEL as well
* Added example custom user that gets tested for Django >= 1.5
Move django-oscar-testsupport into oscar.test

This allows updating code in both places, if necessary. Getting Django
1.5 support would be painful otherwise. This also removes the circular
dependency of django-oscar-testsupport on Oscar.

The requirement in requirements.txt is removed. The imports were updated
accordingly. Unused imports in the touched files were removed. No
further changes.
Update ClientTestCase for Django 1.5