Revert "Moved Oyster-specific behavior into django-stripe-payments because subclassing was causing problems with Django's custom user model"

This reverts commit ce78cca036ff6c74c87468c26a976db623f951a5.
Revert "Merge branch 'python-3-and-django-1.5-compat' of https://github.com/pydanny/django-stripe-payments"

This reverts commit 53525a90a95acb4531335afdceb2f8d5d27bc971, reversing
changes made to 2c36570d79891709f7700ed1ca7991006cbb614b.