Moved GameJoinModule back to being a module for games.  The 'args' parameters for the #addPlayer(Player, String...) method can be used for the player specifying a GameInstance to join.
Added a setPlayerGame method using a registered game's class.  No need to pass around a reference of a registered game, especially if it's the bare minimum implementation and offers nothing other than the name, version, and author(s) of a game.
Added a way to cancel Gamegine from executing its own logic when the addPlayer method is called.
Added a game module for removing players from a game.