Third stab at group support in webapi.

1. I can successfully create a group with a list of users (a json encoded string).  I couldn't figure how other webapi methods consume lists of things... so I opted to use json.  It seems like Djblets/Django consume ',' or ' ' separated lists but that seems fragile.  Using json seemed like the most robust solution.

2. Added a `create` and `update` method that create or update a group, respectively.  It is an error to create a group that already exists.  It is an error to update a group that does not exist.