Add support for removing and getting collaborators

[kevva]

It might be useful to use this module for more than just adding collaborators. I'm not sure if you we should allow for removing to be used in conjunction with addToAll and addToSources though. Could just take a user and an array of repos.

Also, a .get or .check method should be added to get all collaborators of a repo. Should it take more than one repo though?