code: function getRotation(start, end) {
      // need two fingers
      if (start.length >= 2 && end.length >= 2) {
        return this.getAngle(end[1], end[0]) - this.getAngle(start[1], start[0]);
      }
      return 0;
    }
docs: calculate the rotation degrees between two touchLists
  @param {Array} start array of touches
  @param {Array} end array of touches
  @return {Number} rotation