Stopped treating checkout:index as a special case.
This view has a form that assumes anon checkout is allowed, which it
isn't always. Treating it as a special case was causing it to be
displayed when it shouldn't.
Added ability to customise decoration of view functions.
This uses a few new methods in oscar.core.application.Application which
enable custom decorators to be set for given view fns.
The main use case at the moment is to make it easy to ensure that
checkout is not available to non-logged-in users when
OSCAR_ALLOW_ANON_CHECKOUT is set to False.