Add save/restore game

@gabrielcirulli up to you if you want it, I suppose it makes the game 'easier' but avoids some drudgery of getting back up to reasonably high level, and is particularly nice when you can't finish a game on mobile device, as they often reset when waking up from sleep.  Feel free to clean up or request changes, this was a quick night's work as I wanted it for playing myself, sure code could be cleaner but does the job for now :-)