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
added alternative batching kernel (slow)
tweaked controls a bit
added command-line options --selected_demo=<int> and --new_batching
started looking into parallel 3d sap