Simulation of rigid bodies is shitty

[joschu]

Closely related to #6 (centroids). Boxes are OK, but anything with a mesh is really bad. We probably also need some parameter tweaking, e.g. margins, damping, collision flags. It would also be nice to consolidate Jonathan's work on rigid body grasping. It's probably worth looking again at the OpenRAVE bullet plugin--apparently they've been working on it.