add sphere2.urdf move btSpatialAlgebra into LinearMath remove some warnings, introduce BT_ZERO, BT_ONE, BT_HALF as defines for 0.f/0., 1.f/1., 0.5f/0.5 respectively
improved the new demo testbed (work-in-progress) add basic Lua demo, import URDF test, STL import, obj import