Add travis CI

Hey there,

I'm not an expert on these things, but I figured out that automatic tests aren't the worst thing in the world. If you give me a hint how you invoke the coverage test (`nosetests --with-coverage` includes libmproxy.contrib stuff for me), I'm happy to include that info as well. If you don't like Travis CI and have a better alternative, let me know as well! :-)

Travis Integration Guide: http://about.travis-ci.org/docs/user/getting-started/

Cheers,
Max