add 'fixed' joint for btMultiBody
improve btMultiBody version of URDF reader (still work-in-progress)
enabled planar joint for btMultiBody (untested)
enable loading from relative path for .stl meshes
fix compile issue
fix two compile errors, thanks to Travis
https://travis-ci.org/bulletphysics/bullet3/jobs/33858712
more bug fixes
Merge pull request #223 from erwincoumans/master

add 'fixed' joint for btMultiBody