the count of all_products. Tests included

This commit addresses issue #1253.

Currently, the offers app doesn't really consider child products;
behaviour is undefined. This commit addresses the fact: for now, we
assume that child and parent products are tightly coupled, and forbid
child products in a range. You can only add the parent product (and
with it all children).

This requirement might be lifted in the future, but that will mean
additional complexity and we will see if it's necessary.

Closes #1253.

* Removed unused _save method. It's not called anywhere.
* Simplified comment and logic for display_order
* Reordered imports
* Used items() instead of six.iteritems() because create_range does not
  get complex kwargs
* Removed a duplicate test
* Test for the correct exception

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.