Django 17 migrations

OK, This is a re-do of the work done by @jsma in his PR #147, which should now be closed. Credit goes to @jsma not only for the initial work here, but for also key PRs to the CMS project to allow this to happen.

This uses the migrations strategy that is currently in use by CMS and Filer already, using the migrations directory for South users and migrations_django directory for Django 1.7 migration users.

To use this the project **must** be using the latest 3.0.x commits from CMS (currently commit id: ec19aaf041), as the migrations here reference migrations from there. The project should also be using the latest commits from django-filer.