use DIRECTLY_UPDATE_VELOCITY_DURING_SOLVER_ITERATIONS by default for now,
until we find the issue with some failing test cases in btMultiBody
fix a crashing issue in MyMultiBodyCreator.cpp (uninitialized variable)
disable excessive debug printf in URDF2Bullet
add example description for all examples (with word-wrap)
add the VoronoiFractureDemo, note that the collision are disabled after breaking constraints.
add optional GwenOpenGLTest, to make it easier to see Gwen user interface features.
