Add Travis CI to give quicker feedback on pull requests.

There are a few things I'm not entirely convinced about in this, most notably having to use LD_LIBRARY_PATH for it to find the library. However, having CI giving feedback on pull requests is typically a win in my experience, as it makes people realize they've broken things when they otherwise wouldn't have!

See http://about.travis-ci.org/docs/user/getting-started/ for more setup on Travis CI needed before merging this. (Ideally this merge would be the first thing after enabling it.)