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
If container width is 800, and columnWidth is 120, then there are only 6 columns, not 7. Using round would round up to 7.