#6 Network sync

[Ronnie Hedlund]

The game is slightly out of sync for all players.
No lag compensation or other compensation technique is currently used.
This also results in slightly different replays for different players.
If this is fixed, it also solves the other problems.

It should be easier to fix this now when the physics run at a fixed rate (timestep). Some sync is needed to keep the differen clients to run the same timestep at the same point in time, and the ability to run the physics from the timestep a message was sent to the current timestep when the message arrives.