Set reasonable values for yAccel.

I found that a 2:-1 ratio of gravity to hold-jump is pretty good. To
make the whole arc "faster", I increased the initial jump power
slightly, and with it, increased gravity and the hold-jump power. I
think I'm pretty close to what I want the jump arc to look like, though
it still needs fine tuning.

Haven't updated pogo with these new values yet, so pogo is pretty
worthless right now.