Add goal

[gkjohnson]

Fix #14
Fix #24
Fix #28

TODO

• Use quaternions for targets when 3 euler angles are used for targets
• Add example for goal joints
  • understand stall in goal example
  • understand goal not solving for rotation
• Tests for goal-specific functions
• Update Example
• Update solver
• Add makeClosure function that can only be used instead of addChild
• Update WorkerSolver serialization

Issues to Update / Make

• Serialized / buffer data for goal is taking up more memory than is necessary -- just needs DoF
• Look at approaches for using exponential maps for rotation deltas
• Add issue for updating goals to conditionally use euler angles vs quaternion in the target vector