Replace JODConverter by a more resilient solution

We know we have issues with JODConverter :

• soffice process stuck into memory
• soffice not starting
• soffice exit during conversion and never restarted

In addition, there are more and more difference between LibreOffice and OpenOffice.

Possible replacement options are :

What we could do is :

• drop support for OpenOffice and keep only recent versions of LibreOffice
• switch to commandline interface for LibreOffice (use `--convert-to` switch)

This means that every conversion will create a dedicated soffice process, but :

Use CommandLine --convert-to

This may mean drop old version and only recent versions of LibreOffice
Using command line will start a process each time but :

• we don't have uno dependencies
• we don't have the risk of a stuck server

use an other wrapper like unoconv

http://dag.wieers.com/home-made/unoconv/

use LibreOffice java API

http://api.libreoffice.org/examples/examples.html#Java_examples