Apply patch to work with custom user models

Applied from
https://bitbucket.org/prjts/django-registration/commits/ba31fc3053bfca7eb7a19d912882e50e295adc55

Paul Walsh committed 2013-04-19
Fixes *required* to make django-registration work with a custom user model
that subclasses AbstractUser, in Django 1.5. This breaks Django 1.4
compatibility in demostratiing how the current Django 1.5 compatibility
of django-registration is flawed, when actually using the customer user
model of Django 1.5