Multiplayer

Multiplayer would be nice. There is a simple node.js+socket.io based proof-of-concept, but that has the server only relaying positions of the players. In order to support NPCs, general consistency and deter cheating, the server must play the game and send outcome to players. I.e. clients only send input. This also means real-time, but that is not a big problem, the PoC already can handle that.
