Simplify implementation.

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.

This change introduces a templatetag which assigns context for adding a
'back to search results' link to a product detail page.  This can be
useful for certain types of user (who don't use the back button) and
want to get back to their, possibly complicated, set of search filters.

Fixes #456

Fixes #376

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