Before this change, it was conflating WebTest functionality with the
Django test client. This change removes the Django test client
functionality (namely, the login function which was being automatically
called).

A shed-load of tests are adjusted to work with this change.

This almost completes the work for #826 - only a couple more references
to the Django client are left in the test codebase.

Previously, the test suite ran with a custom user model for Django >=
1.5, but it didn't subclass Oscar's AbstractUser.  This commit adjusts
the custom user model to subclass Oscar's so we can test functionality
within the custom user class.

This does require a change to the way tests authenticate as we don't
actually use the username field.

Analogous to the settings for the basket cookie, the cookie that stores
the recently viewed products is now configurable.

Fixes #1064

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