Add support for tox (http://tox.testrun.org/) and Travis CI (http://travis-ci.org/)

tox: http://tox.testrun.org/
Travis CI: http://travis-ci.org/

You can see the Travis CI status of my fork (passing! :-)) here: [![Build Status](https://secure.travis-ci.org/msabramo/jinja2.png?branch=testing)](http://travis-ci.org/msabramo/jinja2) -- http://travis-ci.org/#!/msabramo/jinja2