DART 4.0.0

• Added implicit joint spring force and damping force
• Added planar joint
• Added soft body dynamics
• Added computation of velocity and acceleration of COM
• Added bullet collision detector
• Improved performance of forward dynamics algorithm
• Improved dynamics API for Skeleton and Joint
• Improved constraint dynamics solver
• Improved calculation of equations of motion using Featherstone algorithm
• Improved optimizer interface and added nlopt solver
• Fixed self collision bug
• Fixed incorrect integration of BallJoint and FreeJoint