Custom user compatibility layer for Django 1.5

Hi,

I have added a simple compatibility layer to use the new custom user introduced in Django 1.5.

Quick note: I have also added a TestCase class in **tests/base.py** to provide a default client which is used in your tests, apparently **test_utils.TestCase** redefines **_pre_setup** and does not redeclare a client like [here](https://github.com/django/django/blob/master/django/test/testcases.py#L465).

Let me know if you want further information or elements to include my PR.