WIP: AUTH_USER_MODEL support

* only works if the custom user model is also called ``User``
* currently we run the whole testsuite with a custom user model, because that is more likely to fail than with the default user model. alternatively we could run the testsuite multiple times: http://stackoverflow.com/questions/15369987/testing-with-a-custom-user-model-as-a-foreignkey-in-django-1-5
* needs docs