Can't extend AbstractUser within customised customer app

According to Django [documentation](https://docs.djangoproject.com/en/1.6/topics/auth/customizing/#custom-users-and-signals) we cant' use `get_user_model` in [this](https://github.com/tangentlabs/django-oscar/blob/master/oscar/apps/customer/alerts/receivers.py#L9) context.

Related Django bug - https://code.djangoproject.com/ticket/19218.

I'm still thinking it's a bug in Django, but it's always easier to document the limitations. :)