Add unit tests for version_info.get_version module

[piotrkilczuk]

• write the tests for every conditional
• add coverage and aim for at least 80% if not 100%
• add coverage info to README.rst:

.. image:: https://coveralls.io/repos/TyMaszWeb/python-version-info/badge.png
  :target: https://coveralls.io/r/TyMaszWeb/python-version-info

• make all running with nose
• make all running with tox (Python 2.5 to 3.4 perhaps?)
• maybe even add Travis support and a Travis badge