code: function _fromEuler(x, y, z) {
    x = x / 2;
    y = y / 2;
    z = z / 2;
  
    var c1 = Math.cos(y),
        s1 = Math.sin(y),
        c2 = Math.cos(x),
        s2 = Math.sin(x),
        c3 = Math.cos(z),
        s3 = Math.sin(z);
  
    return new _Quaternion(
        s1 * c2 * s3 + c1 * s2 * c3,
        s1 * c2 * c3 - c1 * s2 * s3,
        c1 * c2 * s3 - s1 * s2 * c3,
        c1 * c2 * c3 + s1 * s2 * s3);
  }
docs: Returns a quaternion for the given euler angles in radians
  The rotations for the euler angles are applied in the order: z then x then y
  
  @param {Number} x y z: euler angles in radians
  @returns {Quaternion} quaternion