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.

The profile test can_change_password checked if the password change
form can be submitted succesfully, but did not verify that Django's
authentication system accepts the new password.

Fixes #376

A user in Oscar is identified by email address instead of the
username. This is, however, not set as a ``unique`` constraint
in the user model in ``django.contrib.auth.models.User``. Checks
if an email already exists are carried out when a user registers
but are ignored when a registered user changes their
profile. This can lead to multiple users having the same email
address which should not happen.

I provide a failing test with a mixin that can be used in both
the ``UserForm`` and ``UserAndProfileForm`` to clean the email
field when validating the form. A ``ValidationError`` is raised
when a user with this email address already exists and is not
the currently edited instance (makes sure that profile updates
with unchanged email work still).

Fixes #324

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