Fixes #33 redirect after user registration no longer raises a 404

By default django-registration redirects to user profile page and
there is not way to change that. At this point in the development
we will not be using user profiles. I subclassed RegistrationView,
provided by django-registration, and shadow its get_success_url
method.
Merge pull request #35 from openrural/fixes/#33-user-profile-page

Redirect after user registration no longer raises a 404