Support for django 1.6, reversion 1.8

The 1.5 urls template tag change was the bulk of the issue. 

I also added a patch that handles the case where you have deleted a field from a model and still want to compare. That can be done in a separate pull request, but i needed it for my site so its here. 

Also some very light pep8-ing in a few places. 

Let me know if there is anything I can do to help!