make jump height dependent on horizontal speed

Closes #2

I'm not sure whether this is what you had in mind, @michaelficarra, but it seems to do the trick. I doubt `dy = 2 * abs dx` perfectly replicates the original game's logic, though.

`2` is a magic number here. Let's pull it out as a `coefficientOfJump` constant. And we should also probably have some `minimumJumpHeight` that gets added on so that he can jump while standing still.