Fix Overflow Bug for very big width in IE, replaced Math.round with Math.ceil

I  fixed the bug with very big sizes in Internet Explorer, (getComputedStyle returns negative values).
Because it was already provided an alternative method for calculating sizes in IE, I add a control to use it:
 if (isNaN(returnWidth) || width.indexOf('%') > 0 || returnWidth < 0) { //checks invalid sizes.

Also I fixed a Math.round bug, (math.round rounds values both in excess and in defect), in excess there aren't problems, when defect it clips slides. 
Because of that i replaced Math.round with Math.ceil.