The old Repository class wasn't thread safe as the method instances were
created at compile time.  Since the instance is configured by setting
the basket for each request, this poses thread safety issues as two
requests could share the same method instance (one of which would have
the wrong basket assigned).

This change fiddles with how the methods are handled, ensuring that they
are instanciated within the instance methods so that thread-safety
issues go away.

Shipping methods need a gentle rewrite for v0.7 to ensure they aren't
vulnerable to such issues.

This allows different shipping methods to be handled more consistently
in multilingual sites, where same shipping method will be saved with
different names.

Fixes #850

is_required and sequence_number are removed. That also makes the unused
get_prerequisites function obsolete.

Those fields were added with the idea that those event flows are linear,
but all implementations assume a non-linear pattern.

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.

Should have checked this before pushing before...

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.

Fixes #745

- Add more links
- Add event details to line detail page

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.

As they save stuff...