Building and testing using Travis CI

This PR adds support to build and test MacPass using Travis CI. I've also added a build status image in the README.

You'll see that the build is currently broken, that's because there are some broken tests.

Maybe after fixing the tests we could configure Travis CI to upload the app to [Github Releases](http://docs.travis-ci.com/user/deployment/releases/)? I think we'll also need to fix the signing part if we want to distribute the generated app.