Split into unit, integration etc.

Use charge_incl_tax instead of basket_charge_incl_tax().  This is
clearer.

Backwards compatible versions of the old methods are kept for now.

Related to #805

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.

The backported version didn't explicitly create the ProductAttribute
instances before assigning and so they didn't save.

This makes creating simple shipping repositories simpler as you only
need to override the methods attribute.

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.

shipping/models:ShippingMethod was missing the is_discounted attribute that
exists in the de-facto superclass in shipping/base:ShippingMethod.

Also added a few comments to clarify how they relate to each other, and
tests to check if is_discounted is present on various instances of
ShippingMethods.

Fixes #376

Also split them into unit and functional tests.

This is used by the weight-based shipping method but can be used by
others too.