Fix footnotes markup

The markup generated does not match PrinceXML format (http://www.princexml.com/doc/8.1/footnotes/). This plugin converts the markup generated to the "Prince Format".

The output generated without the plugin: ![original](https://f.cloud.github.com/assets/184187/434539/a9f1db98-af4d-11e2-9e69-ab348a2fe9ba.png)

The output generated with the plugin: ![patched](https://f.cloud.github.com/assets/184187/434550/c5e50a1e-af4d-11e2-82cd-f5c3fde14239.png)