It only prevents adding a product to a range; offers can be applied
nonetheless (despite the docs saying otherwise). The same functionality
can be achieved by overriding get_is_discountable, so it can be safely
removed.

This prevents an IntegrityError from slugs clashing after renaming a
range to avoid the checked-for name clash.

Fixes #1424

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 might be futile.

Fixes #376

They weren't applying to lines properly.  Fixed with test.