added DrawerClock class

This class represents the time unity for the map. It is instancied
started by the map. It will call the update method from map to update
the screen. We needed the DrawerClock class to make sure that no
dependencies link the graphic part and the AI/behavior/calculation part,
we ensure that the drawings are scheduled by an other object than the
entities themselves. But it is possible that this class offers more
utilities than the one we actually use so discussion about it is needed.