Add support for testing with tox (http://tox.testrun.org/)

This will allow test across multiple Python versions - e.g.: 2.6, 2.7, and 3.3.

The motivation for this is I want to tweak `rxjson` to be compatible with Python 3 as this is  needed for projects that depend on `rxjson` to work with Python 3 (e.g.: [cornice](https://github.com/mozilla-services/cornice)).