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)

Includes PR #1148 by @stefanw

#### Other pull requests
PRs, that tried to partially address of Django 1.7 support: #1179 #1170 #1153 (not part of this PR, just for reference)