add jumpSpeedMove option

you can test it at http://maxogden.github.com/ludum-dare-26

p.s. I dunno if this is the best way to achieve this...

my problem was that movement in the air was too frictionless and made it hard to accurately jump on top of things. this fixes the case where you are standing still and you want to jump up onto something but doesnt yet fix the case where you are running and jump (you still retain high velocity in that case)

it'd be nice to get some more "game feel" specific hacking done on voxel.js soon