Fixes layout calculation problems for Firefox and Safari

In Firefox the this.gutter value is sometimes a bit different. That causes in some use cases a this.cols calculation result lower than the expected. Using Math.round instead of Math.floor fixes this problem here and outputs the correct this.cols value.

There was a problem in calculation of the colSpan:
Firefox: operator % produces in some cases results > "1"
Safari: operator % produces in some cases "1" as result
This calculation represents the fractional difference between `item.size.outerWidth` and `columnWidth`, not the pixel measurement.