Added an initial tox config for easier local testing

This PR makes use of tox for testing. 
With this PR, users on Linux can test CleanerVersion by running 'tox' in the project's root directory. 
Users on Windows (working with a vagrant/VirtualBox setup) will still not be able to use tox, since there seems to be a bug in the FS implementation used by VirtualBox, which does not allow to create hard links, which in turn prevents 'python setup.py install' to terminate properly.