Consumption now sorts the basket lines in descending price order so that
the most expensive products are consumed first.  This means that
multibuys apply in a more natural way.

Fixes #348

The OrderDiscount did not store (a copy) of the offer
name within a separate field which causes an issue when the
offer is deleted. E.g. it breaks the order history view for
the customer because no offer data is available.

I have added a new field to OrderDiscount that is automatically
populated from the offer.name field. This works in the same way
as voucher_code does in the same model.

I also found two issues with the voucher implementation within
the same model that are fixed now.

Fixes #330

* 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.

Added new field to offer that allows their max num applications to be
controlled.  Dashboard adjusted to support this too.

This fixes a bug with adding customised products to the basket, where
the clean method would get a MultipleObjectsReturned exception as the
lookup wasn't taking options into account.

Also fixed a display issue with required options in basket forms.

It's potentially confusing to have it default to a value as you might
miss this in your project.

This involves:
* Better checkout templates
* Updated availability logic for products with no stock
* Updated dashboard templates

This is a major clean-up of offers, tidying up the implementations and
rewriting the test suite.  This should also clean up some edge cases
around the use of max_affected_items.

* replaces all occurences of ``datetime.now()`` with
    ``django.utils.timezone.now()`` which returns aware or naive
    datetimes based on ``USE_TZ``
* increases ``South`` version to 0.7.6 in dependencies to make
    aware datetimes work in migrations.
* Oscar's sandbox is now configured to use aware timezones by
    default

Turns out this is a bit confusing.  Better for it to do nothing by
default and this behaviour can be enabled when it is needed.

Change to products URL was not picked up here.

Now using --spec format which is more readable (most of the time).

They weren't applying to lines properly.  Fixed with test.

Also tidied up test case for basket model

* Less DB queries
* Added basic tests

This avoids conflicts with other libraries that use the same trick for
extending templates.

The validation on the stockrecord form needed to be removed to allow
group products to be created, which don't have stock records.

Also added tests for vouchers

They now must have a title

To work around MySQL bug with fields of > 255 chars not being allowed to
have unique indexes.

Plus tweaks to test method names

Also split them into unit and functional tests.