Deferred benefits could reasonably make use of that information.

This involves some quite involved changes to the internals of how offers
and organised.  A new boolean field has been added to the offer model to
indicate whether to apply to prices exclusive of tax.  This is used
within the offer methods to determine whether a basket meets a
condition, and how to apply the benefit if it does.

Fixes #925
Fixes #924

To support multi-currency sites.  This commit also made the order totals
a mandatory input for creating an order, which meant quite a few tests
had to change to support the new API.

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.