Added a Game Controller class.
 > moved all game controlling methods, data (like maze, undoStack  etc into it)
 > in process, removed the global object map...we don't need it (at-least not now).

Fixed a bug:
 > undo stack was not being reset on start of next level.

Replay Level:
 > added a button to reset the present level.

Included few items in todo list.