Django 1.7 support

Initial support for django 1.7. Fixes #134
@Walkman 

I don't like this 1.6 - 1.7 dance either but I don't want to fork unless absolutely necessary. Forking is not cheap. I'll have to keep the codebase up to date with upstream with maintaining my patches for 1.7. As long as this repo is actively maintained, I'll not try to maintain a fork to support something special. 

What we can do to avoid complexity is to not jump on the 1.7 checks framework bandwagon right away; we keep using the legacy exception raising code until say 1.9 or 2.0 at which point we deprecate support for 1.6 and completely switch to the checks framework.

It is a bit complex but it is limited to the checks stuff, other core parts of the app aren't complicated.

I want to avoid forking at all costs and contribute to the upstream repo as long as possible. What do you think?