Merge pull request #49 from aelkhour/master

src/multibody/joint/joint-free-flyer.hpp

src/multibody/joint/joint-planar.hpp

src/multibody/joint/joint-prismatic.hpp

src/multibody/joint/joint-revolute-unaligned.hpp

src/multibody/joint/joint-revolute.hpp

src/multibody/joint/joint-spherical-ZYX.hpp

src/multibody/joint/joint-spherical.hpp

src/multibody/joint/joint-translation.hpp

src/multibody/model.hpp

src/spatial/motion.hpp