Fixed 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 avoids this situation.
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...
Merge branch 'master' into develop
Made sure new Font objects get disposed after use to avoid memory leaks.