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 
Are there consequences of changing this ?

i.e. do we need to update requirements.txt, setup.py or rpm/deb package dependencies ?