Allowing for slug filtering to be in boundaries.

This fixes #42 and allows for any filtering on slugs, so I can do something like:

http://localhost:8000/1.0/boundary/?slug__in=2715101864-minor-civil-division-2010,2713702890-minor-civil-division-2010

I am not sure if this should really go upstream, but I don't have the knowledge to override this sort of thing for our instance only.