Improving uno.py code style

* Fixing pep8 violations
* Improving comments, docstrings, and unifying coding style
* Using functionality copied from six library for Python 2/3 compatibility
* Using standard traceback formatting

Change-Id: I62bd0e8513ffc59202163002fa4adea3d92572c3
Reviewed-on: https://gerrit.libreoffice.org/22848Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>