Add a customized version of django-registration

The current version of django-registration, 1.0, does not work with custom
user models. There is a small patch to make it work:

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

We cannot directly fork the repository because it is Mercurial, so I
downloaded the source and created a new github repo

https://github.com/azavea/django-registration

I applied the patch to this repo and added a reference to it to our
requirements.txt

The audit system prevents calling User.save directly, so I needed to write
custom backend view functions in registration_backend/views.py and
create an associated url set in registration_backend/urls.py

The registration templates were pulled, unmodified, from

https://github.com/macdhuibh/django-registration-templates