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

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

* They are now known as 'stock alerts' instead of notifications.
* They have been moved into the customer app largely (instead of their
  own one).
* The model implementation has been simplified.

This is a squashed, rebased version of the original branch.

Original commit messages:

* product notifications are visible *only* if a product is not in stock
* or does not have a stock record (a new product).
* an anonymous user can sign up for a notification of a product. They
* receive a confirmation link that they have to open to activate their
* sign up. An anonymous user does not have any means of managing their
* notifications in a list. The confirmation email contains a second
* link, however, with a *unsubscribe* link that allows for disabling of
* the notification.
* a registered user can sign up for notifications without having to
* confirm it. When a registered user signs up for a notification, the
* "Notify Me" button disappears from the product information and is
* replaced by a note stating that the user has already signed up.
* registered users can manage (activate/deactivate) their notifications
* in their account settings.
* an anonymous user that has signed up for notifications and creates an
* account will pull in their notifications and have them assigned to
* their account and are then no longer marked as anonymous
* the notifications app registers a receiver for the ``post_save``
* signal of ``StockRecord``. Whenever a stock record is updated, the
* notifications are checked for this particular product and emails are
* sent out. These notifications are then disabled (not deleted) and
* marked with the date the email was sent. This hides the message from
* the registered users account. Notifications are still accessible,
* however, for staff members in the dashboard
* In the dashboard, the ``Customers`` navigation node is extended with a
* ``Notifications`` child that allows for editing, deleting and viewing
* all notifications. It also provides filtering capabilities for them
* based on status, customer name, customer email and/or product keyword.