Tests should now pass.

Introduced get_primary_address and get_address_for_stockrecord. I opted
to implement rudimentary logic supporting only one PartnerAddress
instead of raising NotImplemented. I'm assuming that given the demand
for multiple partners scenario, it is more likely than not that a user
will need to store an address for a partner.

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

* Fixes bug with prefetch_related where looking up secondary relations
  raises a DoesNotExist exception.
* Reworks cached partner map to use PKs instead of names.
  Availability logic from product class is looked up in product obj
  rather than in wrapper to avoid unnecessary queries.

It's potentially confusing to have it default to a value as you might
miss this in your project.

This involves:
* Better checkout templates
* Updated availability logic for products with no stock
* Updated dashboard templates

Turns out this is a bit confusing.  Better for it to do nothing by
default and this behaviour can be enabled when it is needed.

Also split them into unit and functional tests.