Allow use of native migrations in 1.7

First pass at #1559. 

South provides a [SOUTH_MIGRATION_MODULES](http://south.readthedocs.org/en/latest/settings.html#south-migration-modules) setting. We use this to move the existing migrations to one side. 

1.7's `makemigrations` is responsible for the new migration. 

Adding the South setting to `rest_framework.setttings` doesn't work — and (of course) there's no reason that it should. 

But manually adding it to your own settings.py __does work__. The following works:

```
mkvirtualenv drf-1.6
pip install -r requirements.txt 
pip install South
django-admin.py startproject playgound
cd playground
ln -s ../rest_framework/ rest_framework 
# Add rest_framework, rest_framework.authtoken, south to settings.
# Add SOUTH_MIGRATION_MODULES setting
./manage.py syncdb
./manage.py migrate --list # Just the unapplied authtoken migration
./manage.py migrate --all
```

I need to back out the change to `rest_framework.setttings` and document (where?) the change and (at first glance) it looks good. 

Thoughts?
Shouldn't that be added in the 2.4.x line as this will go against the stability we'd like to have with django rest framework ? @tomchristie your toughs ?