StereoEffect - Modified to be an off-axis stereo effect

I changed the StereoEffect to perform a more proper off-axis stereo as explained here http://paulbourke.net/stereographics/stereorender/

Stereo effect now supports both `eyeSeparation` and `focalLength`

I made a small demo where there are a lot of changes in eye separation and focal length, may be a bit dizzy. Demo here:
http://fonserbc.github.io/ballbox/


I was doing something for Cardboard and I found that the StereoEffect given was just a simple displacement of the cameras, I have worked with stereoscopy before and I thought it could be useful for other people.