code: function getDistance(touch1, touch2) {
      var x = touch2.clientX - touch1.clientX,
          y = touch2.clientY - touch1.clientY;
  
      return Math.sqrt((x * x) + (y * y));
    }
docs: calculate the distance between two touches
  @param {Touch}touch1
  @param {Touch} touch2
  @return {Number} distance