Add support for tox

Tox allows you to run the `nosetests` test suite in multiple Python versions (py26 and py27) and helps to ensure that both work.

Speaking of which, a few tests are broken on py26, because they're using unittest assertions that don't exist in py26 (see https://gist.github.com/msabramo/5e82d78ad9687ddf2cbc)