Added Django 1.7 support

Moved old migrations to south_migrations and updated the docs to reflect
Created new 1.7 compatible migrations (including setting dynamic choices/defaults correctly to avoid triggering migrations if users override the defaults).

Full support also requires upstream to merge some currently open pull requests first since django-cms and django-filer are not quite 100% Django 1.7 compatible. See:
- https://github.com/nephila/django-filer/pull/1
- https://github.com/divio/django-cms/pull/3500