Improved throttle and gimbal controls

The timestep was giving me a hard time (energy wasn't conserved the way
I was going it), so I've tabled the throttle/gimbal physics for now.
Instead, I improved the incrementing of the throttle/gimbaling, and
added keys to cut/max throttle and center gimbal.  I'm using the "," and
"." keys since they feel nice and are close to the arrows, but those are
kind of weird keys to use for controls.  Feel free to change them to
something more conventional, if you want.

I've commented the code so it should be easy to see what's going on.