Merge pull request #6 from leapmotion/stack-unstack

Stack unstack
High-speed intersection-starts [step 1]

This is an improvement.  However, there are still some issues,
especially at low FPS:
 - Drift within image
 - Can drift when entering. (may be the same issue as above)
Some cleanup, and add a todo
Upgrade to 0.6.4 and clean up render flow
Clean up phantom VRCursor clicks
Fix issue where resize would not actually resize renderer
No longer track thumb proximal
Add basic testInteractablePlane example
Add some inline docs on contstrainMovement
Draft testButton example

allows pushing button. This includes an optimization for x and y w/o z,
and vise-versa.
Some getZReposition Cleanup
Refactor to allow z-dimension physics
Add in restoring spring force
Refactor movementConstraints to return corrected values

 - also, only one constraint per, so we don’t need any iterative
solving BS
Add button example
Update constrainMovement docs