Pull in a newer version of six and use it for metaclasses.
The version of six shipped with Django 1.5 is missing some important

compatibility primitives, like the add_metaclass decorator. I've pulled the

copy shipped with Django 1.6 into our old util/compat directory (which was

empty as of a few days ago) to use until we bump our dependency.
For now, this is used to handle the extension hook metaclasses.

Ran djblets unit tests
Ran Review Board unit tests