fix 1093122 - Convert to Django 1.7, migrations

This builds on PR #14 - merge that first.

This PR fixes [bug 1093122](https://bugzilla.mozilla.org/show_bug.cgi?id=1093122):
- Drops support for Python 2.6
- Upgrades from Django 1.6 to 1.7, including pre-release versions of some requirements for Django 1.7 support
- Upgrades other dependencies to latest
- Adds 1.7-style migrations
- Other cleanup



1.5.3 contains the PRs so we should go back to it