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?
I don't think this will change anything for those two points. We are just using an alternative location for south's modules.
However I don't know the minimum south version that will support that configuration option.