Indicate non-coverage areas

I marked the lines we don't plan on hitting in the primary test run (Django 1.7 on Python 3.4) with the 'no cover' coverage pragma and marked the reason for the compatability: The version of Django if that was the reason, or a short description if it was something else.

I also removed the compatibility we had for Django versions older than 1.4 that did not have the timezone util module.