instead of send the room object just send the is of the room
added ErrorMessage class for error message constants to be used by
various exception classes
added RoomManagerException
made userList private, added methods to add, remove and check user from
the list
added Group class to group rooms by category
added methods to add and remove a user to a room, the add method is
implemented with checks to see if the room is full and if the room
already contains the user