Started to implement save menu, run into some errors.

- Using built-in module "shelve".
- Added save(), load() methods to Game class.
- save_game.dat, save_game.dir and save_game.bak are files created
automatically to store the values.
- Getting KeyError when loading game state from files.
Merge pull request #24 from MihaiPro/Save-Game-State

Started to implement save menu, run into some errors.