Enforcing SECRET_KEY

Hi,

I'm totally not sure if this is good. I ran the tests and they pass. Do note that one problem is if SECRET_KEY isn't defined at all, which the code tries to check, but I can't test for it because Django's `override_settings` doesn't seem to allow deleting a setting. (Submitted bug report here: https://code.djangoproject.com/ticket/20032#ticket )