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.

This paves the way for custom benefits to return successful applications
that don't lead to a discount.

This might be futile.

* Includes a migration for the proxy_class field
* Dashboard templates needed adjusting

Fixes #386

Renaming column for clarity now that we have alternative fields for
tracking consumption counts

Fixes #376

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