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 (fingers)
  no scale is 1, and goes down to 0 when pinched together, and bigger when pinched out
  @param   {Array}     start
  @param   {Array}     end
  @returns {Number}    scale