Fix for https://github.com/KhronosGroup/OpenCOLLADA/issues/207
alternative fix for https://github.com/KhronosGroup/OpenCOLLADA/issues/207
Added comment to SkinControler fix
skin controller: take care of Joints referenced by Id/Sid
skin controller: move validate and add joint to own function
Merge pull request #209 from gaiaclary/master

Fix for https://github.com/KhronosGroup/OpenCOLLADA/issues/207