Zero gravity bouncing.

You will bounce when there is no gravity. Fixes #3628 I guess.


![gravity_bounce2](https://cloud.githubusercontent.com/assets/1524810/3222040/1073ad96-f012-11e3-92d6-fbc2ea45baa3.gif)
![gravity_bounce3](https://cloud.githubusercontent.com/assets/1524810/3222041/156278c8-f012-11e3-993b-0c8378c4a2bd.gif)
![gravity_monkeys](https://cloud.githubusercontent.com/assets/1524810/3222042/180e192e-f012-11e3-8b7a-b655bfbc40f2.gif)