Add tox and Travis CI

This pull requests adds tox (http://tox.readthedocs.org/en/latest/) and TravisCI config.

Tox is a Python library which makes running tasks such as tests under different Python versions easier. Travis CI config just runs (atm) unit tests on every commit.

Ideally this pull request would get merged and TravisCI hook enabled for this repository.

Running tests under Python 2.6 also discovered a couple of issue which I will fix in a separate pull request.