tox.ini to run python tests in isolated env

Define a basic env which simply runs nose using python 2.7. setup.py is
not used thanks to tox.skipsdist = True

This will let us add jobs on Wikimedia Jenkins.

Example usage:

 $ tox -e py27
 <tests are running>

Change-Id: I57ff7fa1fe344713e54e547c9fee252838d47ae9