supporting openoffice fodt

```
paepke:
I'd like to request to support the openoffice flat-xml format. It would be good
to have this format cause of the better support to have openoffice files under
a version control system not only as binary blobs like the standard zipped .odt
format.

ced:
What is the constraint of fodt?
I tried to save a file into fodt but it failed.

paepke:
On some OpenOffice?.org versions that filter isn't installed.
Sorry for that long delay in answering, but it seems i don't get informed by
trac.

udono:
It seems that libreoffice has fixed this issue:
http://www.libreoffice.org/assets/Uploads/EN-Project_images/NewFeatures/General/
1000X750/save-flat-xml.png
http://www.libreoffice.org/download/new-features-and-fixes/
```

Original issue reported on code.google.com by `cedric.krier@b2ck.com` on 8 Jul 2013 at 4:46

ファイルやテストは特定されていません。まず、python-relatorio がドキュメント形式をどのように選択して処理するかを確認し、次に想定される OpenOffice の flat-XML 動作と LibreOffice との互換性を検証してください。明示的に定義されたとおりに FODT の入力または出力がサポートされ、テストでカバーされれば完了です。

索引モデルが issue の本文から書いたものです。