Added Reset shortcut and out of screen indication

Dear players, 

It came faster than I thought, but that's it, the reset shortcut on the key "R" is implemented.

And there is now a small indication on the top of the screen when a stone is out of the screen to locate it, and have a better idea of where it will land. 

The game won't change anymore at this point, we may add in the future, some kind of a scoreboard, and maybe we will add other levels, but this will be only if a lot of players play it and request it. 

Enjoy ;)

Charlie and Clément