Timers

This separates the physics timer from the request animation frame timer per @mikolalysenko's advice.  

It adds `game.pin(target[, updateEvery][, objectTarget][, "optionalName"])` for easier debugging.

It fixes the issue with infinite chunk loading by reinstating the `position` event (this time, for whatever avatar the controls are pointing at).

It adds `game.paused`, which keeps the game from firing physics ticks until it's `true`.