Add support Django 1.6

Branch tests agains Django 1.4 up to and including 1.6.

Most of the work in this branch went into fixing unittests for okconfig since the previous test runner only ran tests for apps in settings.INSTALLED_APPS but now uses autodiscovery, ref: https://docs.djangoproject.com/en/dev/releases/1.6/#discovery-of-tests-in-any-test-module

Closes #415 
Affect #416 and #407 
TL;DR, no.

I believe there are no consequences. The json module was added to stock python in version 2.6 and it does everything we need. https://docs.python.org/2/library/json.html

We could remove simplejson usage everywhere and remove references from build requirements but I think that belongs in a different changeset.