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.

Testing done:
* Ran djblets unit tests
* Ran Review Board unit tests

Reviewed at https://reviews.reviewboard.org/r/4922/