Improve the game loop

It can now skip frames. Replays are a *lot faster* on the "max speed" setting.
You're right, I'll add some comments. Meanwhile, there's a very nice explanation [here](http://entropyinteractive.com/2011/02/game-engine-design-the-game-loop/) which is where I got the inspiration for the `minUpdatesPerSec` thing (see the "More Advanced Game Loop" in the linked article).