RotationAxis incorrectly calculated

[pehrlich]

    this._rotation[7] - sinceHand._rotation[5],
   // should become
    rotation[7] - rotation[5],

where rotation is defined by

    Leap.mat3.multiply(rotation, basis, basisLast) 

https://github.com/leapmotion/leapjs/blob/master/lib/hand.js#L278-L282

As it is right now, the axis of rotation has a slight mis-alignment, which grows larger with faster rotations.

Also, Axis angle should use atan2 instead acos2, to prevent possibility of singularities when standing still:

axisAngle = function(rotation) {
  //NOTE: Index = 3*row + col when basis vector 
  var axis = [
    rotation[5] - rotation[7],
    rotation[6] - rotation[2],
    rotation[1] - rotation[3]
  ]; 
  var sin = Leap.vec3.len(axis);
  var cos = ((rotation[0] + rotation[4] + rotation[8]) - 1.0)*0.5;
  var angle = Math.atan2(sin,cos);

    if (-1e-6 < sin && sin < 1e-6) {
        axis = [0, 0, 0];
        angle = 0;
        return [axis, angle];
    }

  Leap.vec3.scale(axis, axis, 1 / sin);
  return [axis,angle];
}

cc @GabrielHare

PS: full fiddle is here: http://jsfiddle.net/kLs8ymye/2/