Keystone v3 improvements.

Managers for project group membership, project user membership, project combined membership (users and groups); for group project membership, group users, group roles; for user project membership, user roles.
The thing I don't like is that a person using this API needs to know this term "assignable" in order to use it. I realize the lack of documentation/discoverability is largely my fault, but I've tried to avoid that by making the names of arguments simple and easily guessable.

Further thoughts?