Add continuous integration via Travis CI

Now that we have some unit tests, it seems like a perfect time to add continuous integration.

Travis CI is a great service for open source projects, and offers excellent [Node.js unit testing](http://about.travis-ci.org/docs/user/languages/javascript-with-nodejs/).

Travis allows us to put a nifty [build status badge](http://about.travis-ci.org/docs/user/status-images/) in our Readme so that anyone visiting the repo page in Github immediately knows the build status of the master branch.

By default, all branches will be [built when pushed up](http://about.travis-ci.org/docs/user/build-configuration/#Specify-branches-to-build), and [notifications](http://about.travis-ci.org/docs/user/notifications/#Notifications) will be sent out regarding the build status of each branch.

@bcoe Since you're the repo owner, you'll need to [setup Travis CI](http://about.travis-ci.org/docs/user/getting-started/#Step-one%3A-Sign-in) before it'll work for this repo. Note that Step 3 has already been done, and is a part of this PR.