Add ability to scale time (0 = paused, 0.5 = 50% speed, 1 = normal, 2 = 200% speed)

[ericlathrop]

This will let us nicely pause the game, but has the benefit that you can slow down or speed up time for neat effects.

In order for this to work, we'll need the ability to run multiple scenes at the same time, because currently if you set the time's speed to 0, then the simulation won't run again, so your code won't have a way to un-pause it. You'll also want a second scene to run so you can draw a menu screen on top of a paused game.