Make tests pass in Python 2.7, 3.2, 3.3 and 3.4

This change makes the tests pass in multiple Python versions, including Python 3.x. As `subprocess.check_output` returns bytes the equality checks needs to compare bytes as well.

This change also add a configuration file for the tox tool so that the tests can easily be run in various Python versions. You can run them with ``pip install tox`` and ``tox``.