Add support for python3

Libreoffice 4.0 will switch its internal python version to 3.3.0
so it's to support that.

Porting done automatically 2to3 plus print_function import added
manually. Tested on both libreoffice master with internal python
and with libreoffince 3.6.4 on debian with system python 2.7.

This bumps the minimal python version to 2.6 since 2.5 does not
have the print function.
Fix typo: "posible" -> "possible"
Fixing Issue #120 (When converting odt to pdf table of contents may have wrong page numbers)

Issue: https://github.com/dagwieers/unoconv/issues/120

I added cycle to update indexes two times.
Fixing fix for issue #120

indexes.getByIndex(i).update() must be called second time
after second call to
    document.refresh()
    indexes = document.getDocumentIndexes()
Add XLSX support

add the 'Calc MS Excel 2007 XML' (XLSX) export filter
Merge pull request #131 from mondayx/patch-1

Add XLSX support
Merge pull request #117 from ojwb/master

Fix documentation typos
Merge pull request #127 from peterdemin/master

Third attempt to make pull request for fixing issue #120
Merge pull request #111 from xrmx/master

Add python 3 support
Merge in updates from upstream