This allows updating code in both places, if necessary. Getting Django
1.5 support would be painful otherwise. This also removes the circular
dependency of django-oscar-testsupport on Oscar.

The requirements were merged. The imports were updated
accordingly. Unused imports in the touched files were removed. No
further changes.

They need registering using oscar.apps.offer.custom.create_range which
will create the model instance with the correct proxy class path.

After that, they will be available to see in the dashboard (but not
editable) and can be used in offers.

Fixes #385