Zach gameroom invite handler

* Added a new error message: BADARGUMENTS, which is used for example if the gameroom-id is not a valid bson.ObjectId and just a garbage string. This keeps us from 'panic's, which is great!
* Added util.Contains(), which is a method to see if a string is within an array/slice of strings
* Added the actual gameroom invite handler, and gameroom.Invite itself which does the work.