support index number as special shape name

You can specify the shape by index number by whole running number of shape
in the document by "#n" or running number in the specified slide by "#m.n".
These forms are usefull for read-only document or old Power Point 2007.
support Power Point 2007

Tiny support of Power Point 2007.
Note that it does not support Shape.Title property, so you need use
index number form with PP2007.

known issue:
- Size of a PNG image from PP2007 is different to the one from PP2013.
  There's not enough infomation about this problem at this point.
Merge pull request #2 from gotoh/enh-shape-index

シェイプのインデックス番号での指定方法の追加とPowerPoint2007の暫定サポート