`override_settings` used instead of updating setting in `setUp`/`tearDown`.

Currently, if a valid basket line is marked for deletion, any other
invalid basket lines on the formset prevent it from being deleted.

Regression introduced in cca43aa4d1

Fixes #2547.

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

We only needed it because Django 1.4 shipped with a pretty old version
of six. Support for that has been removed, and Django 1.5 ships with six
1.6.1, which is more current than we required.

This nicely avoids an issue with django-extra-views pinning a six
version which caused the sandbox build to fail:
https://travis-ci.org/tangentlabs/django-oscar/jobs/32223978#L971

This is quite a large change. It:

- Alters the add-to-basket URL to have the "base" product's PK in it
- Changes the constructor of the AddToBasket form to not require
  purchase_info (even though I only just introduced it!)
- Renames the product_id field to variant_id and only uses it for group
  products

These changes are motivated by the variant workflow. Before this change,
you effectively had different forms being used for rending and
validation as the product being passed to the form constructor was the
group product when rendering but the variant when validating. This just
about worked but was not nice.

The purchase_info param is removed as we can only calculate the
purchase_info once the variant product has been validated. Hence we
again violate Demeter and make a deep call to the purchase info
calculation within the basket form. Might revisit this again.

This change also simplifies the basket view logic and allows a form to
be removed.

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.

For anon users, the basket ID is stored in a cookie.  Previously, this
was stored with a simple crc32 hash to prevent users enumerating baskets
by changing their cookies.  However, home-rolled encryption is always a
bad idea.  In this case, it may have probably exposed too much
information allowing the SECRET_KEY to be determined.

This change switches away from the home-rolled encryption to use
Django's built-in crypto signing functionality to store the basket ID.

Thanks to Raphael Kimmig for reporting this issue and providing this
patch.

We no longer need to pass the StockInfo instance as the basket has a
reference to the strategy class and can fetch it for itself.  This means
we don't have to break backwards compatibility with Oscar < 0.6.

It now returns a OrderTotal instance which has price fields with- and
without tax.

Hmmm, this turned out to be a monster commit.  This change allows the
basket to be able to correctly calculate prices including tax.

It also requires a whole load of test changes since all baskets now
require a strategy instance to be assigned.

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

Also split them into unit and functional tests.

When merging a cookie basket with a user basket, we now set the new line
quantity to the max of that from the two baskets.  This is to prevent
stock rules being violated by merging these baskets.

This allows us to show warning on the basket page when the price has
changed.

Looks up all shipping methods for a basket and picks the first one to
form the estimate.

tests