Add Travis CI

This is pretty neat. You need to sign in to Travis first (using GitHub's OAuth), then fetch the first commit and push, go to your Travis CI profile, hit "on" for the repo, then fetch the second commit and push again (you need a first push to trigger Travis for the first time, the README adjustment is great for that :)).

http://about.travis-ci.org/docs/user/getting-started/ is a good read.

I've verified it using my fork and it works great. Building with gcc & clang and running the test suite for every pull request ever before even merging sounds awesome to me :)