Validation

This pull request includes basic server-side logic for player movement and physics. Currently, there is no client side prediction or interpolation, which occasionally makes movement laggy or jerky.

Although this feature is certainly not done (eventually we probably want to make it like https://developer.valvesoftware.com/wiki/Latency_Compensating_Methods_in_Client/Server_In-game_Protocol_Design_and_Optimization), I'm sending the pull request now to avoid having a HUGE code dump, and make the diffs more manageable.

Please comment regarding silly things I did, your experience trying out this version, bugs you encounter, etc.