fix colspan/rowspan

[wangfengming]

• fix colspan and rowspan
• self-close <a:rPr/> when no child

[wangfengming]

the first cell has options: rowspan=2 colspan=2

• for cell(0,0): <a:tc> should has rowSpan=2 gridSpan=2 attributes
• for cell(0,1): a dummy cell, <a:tc> should has rowSpan=2 hMerge=1 attributes
• for cell(1,0): a dummy cell, <a:tc> should has vMerge=1 gridSpan=2 attributes
• for cell(1,1): a dummy cell, <a:tc> should has vMerge=1 hMerge=1

[gitbrent]

Thanks @wangfengming !