Clean up PlanarJoint
Parse PlanarJoint in SKEL file format
Remove -fno-inline-functions-called-once option for clang compiler

See: https://travis-ci.org/dartsim/dart/jobs/24079306#L7360
Fix header guard of mixed use of DART_ and SOFT_
Merge pull request #186 from dartsim/planar_joint

Clean up PlanarJoint and parse it in skel file format