improve btMultiBody version of URDF reader (still work-in-progress)
enabled planar joint for btMultiBody (untested)
enable loading from relative path for .stl meshes
enable gtest by default when using premake, added flag without-gtest
use gtest ASSERT_EQ( expected_value, actual_value) instead of the reverse,
this fixes issue 205 (https://github.com/bulletphysics/bullet3/issues/205)