Save move history

[supermitch]

The game should save a copy of the move history as a series of (username, move) tuples. That's all we would need in order to replay the game.

Saving (username, move, state) tuples would mean the entire game could be viewed without having the engine replay the game. That might better.

I think this should be a field in NDB outside of game and state ... a list of json objects where each object is a {'user': mitch, 'move': 'go north', 'state': }. Obviously then we'll need to store the starting state as well.

[supermitch]

Probably mostly finished here: 907367f

But untested. I think original state (no moves) is missing also. Need to add API route to download a game too.