The functionality to have it fetch an existing range instead of loudly
complaining when a range already exists has been added as part of #1053.
But @codeinthehole and I agree that a method called create_range
shouldn't silently ignore an existing range, and can't think of a use
case anymore.

* 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

Rather than forcing text, it's better to raise an error when a uggettext
proxy is used for the range name as this won't save correctly in the
database.

Related to #1053

and unique fields error when creating existing custom ranges.

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.

They need registering using oscar.apps.offer.custom.create_range which
will create the model instance with the correct proxy class path.

After that, they will be available to see in the dashboard (but not
editable) and can be used in offers.

Fixes #385