Django1.7 support

This pull request adds Django 1.7 support. The tests pass on my local machine for Django 1.7, 1.6.5, 1.5.8 and 1.4.13 without error. I have manually tested the Demo site in 1.7 and found no issues.

I addressed all 1.7 deprecation warnings with exception to one and some checks (from the new 1.7 checks framework). I did not address the former because it would have required major changes to the tests or the latter because it might require model changes.