unoconv: new package

unoconv is a tool that converts between any document format supported by
LibreOffice/OpenOffice.

Example of how to convert an .odt file to .pdf:
  unoconv -f pdf some-file.odt

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

Implementation notes:
unoconv must use the same python version as libreoffice (unless it will
not be able to load the pyuno module from libreoffice). And because we
recently switched to libreoffice 4.x, which uses python3, I had to
include unoconv-python3.patch. The patch comes from upstream unoconv.git
repo, so it will be included in the next release.