Better to use filenames which are commonly used in the python word then
custom ones.

We only needed it because Django 1.4 shipped with a pretty old version
of six. Support for that has been removed, and Django 1.5 ships with six
1.6.1, which is more current than we required.

This nicely avoids an issue with django-extra-views pinning a six
version which caused the sandbox build to fail:
https://travis-ci.org/tangentlabs/django-oscar/jobs/32223978#L971

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.

Fixes #376

* Tests now split into more coherent modules
* Added IDs to forms to help WebTest
* Some refactoring along the way