Update SheetUtill.cs to fix memory issue with AutoSizeColumn for large worksheets

AutoSizeColumn would regularly fail in sheets with > 30000 rows. The
call to "new Bitmap(2048,100)" would throw ArgumentException with "Parameter is not valid" (on a system that otherwise low on memory). Disposing of the Graphics object each time helps avoid this.
Update SheetUtil.cs

Fixed error from previous commit.
Merge pull request #8 from kennethahn/patch-1

Update SheetUtill.cs to fix memory issue with AutoSizeColumn for large w...