code: function getScale(start, end) {
      // need two fingers...
      if (start.length >= 2 && end.length >= 2) {
        return this.getDistance(end[0], end[1]) / this.getDistance(start[0], start[1]);
      }
      return 1;
    }
docs: calculate the scale factor between two touchLists
  no scale is 1, and goes down to 0 when pinched together, and bigger when pinched out
  @param {Array} start array of touches
  @param {Array} end array of touches
  @return {Number} scale