add rolling/spinning friction to cube, remove it from plane/samurai.urdf
URDF2Bullet: support joint limits for revolute and prismatic, only if defined (if upper < lower, disable limit)
add some profiling markers to improve performance
add wsg50 gripper with modified r2d2 gripper tip
expose a fudge factor to scale inertia, to make grasping more stable
(until we have better grasping contact model/implementation)