Django 1.7 support

This pull request should pass across all supported python and Django versions, including Django 1.7c3

#### Features
* New migrations, South migrations still supported (via South>=1.0)
* Fixed custom user model handling for new app loading mechanism
* Multiple testsuite fixes (details explained in commits)

Note: django-oauth-plus is not yet 1.7-compatible, so tox uses alternative fork with my patch to support Django 1.7. This is planned to be fixed as soon stable django-oauth-plus is released. However this should not block merging this PR, as it's an optional dependency.

Includes PR #1148 by @stefanw

#### Other pull requests & issues
PRs, that tried to partially address of Django 1.7 support: #1115 #1153 #1170 #1179 (not part of this PR, just for reference)