Add mysql_oursql to south engine_modules

I am looking into migrating from MySQLdb to OurSQL on the count that OurSQL seams to allow gevent monkey patching. The database backend can be found here:

​https://github.com/dcramer/django-oursql

but it looks like south throws an exception if I try to use:

'ENGINE': 'mysql_oursql.standard'

as a database engine. The error stems from the fact that mysql_oursql is not included in the engine_modules aliases from south.db. This patch should fix that.