Feature/message bus

Game now holds a MessageSystem (a thor custom event system), which is passed to units and the map. Now when a unit completes its move it sends a message with the id "unitMove". Map is listening for messages with this id and removes the shroud around the unit.

Unit no longer holds a reference to the map. Checking for whether the unit is on valid ground has been disabled for now; it should be accomplished by messages as well.