Add support for Django 1.6

This PR adds support for Django 1.6. I've taken the liberty to remove the use of simplejson without providing a backwards compatible solution for those on Python 2.5 or earlier. Django 1.5 stopped supporting Python 2.5 and I think it's safe for Respite to do the same.

I've not tested this version with versions of Django earlier than 1.6 and can not guarantee that it works, so I believe the safest thing to do would be to bump the version number and require Django >=1.6.

I have not as in PR https://github.com/jgorset/django-respite/pull/52 done anything to support Python 3.