App labels now show up correctly if string is path to app.

When apps are specified in settings.py as a path, eg somepath.tothe.app or django.contrib.auth, they are unfit to be used in SOUTH_MIGRATION_MODULES. This setting calls for just the label of the app, not the path to it. See http://south.readthedocs.org/en/latest/settings.html#south-migration-modules.