(maint) Add an initial travis configuration

@peterhuene you may have noticed that I flipped on travis, and I was churning on a .travis.yml that I could get to pass. It's not a thing of beauty since the travis build has to install gcc 4.8 and build a newer cmake, but it gives us some coverage.

And btw, I had clang in there originally, but yanked it because Ubuntu 12.04 has an older clang, so we'd need to track down (or build) a newer clang as well. Maybe they'll move to trusty now that it's out, which would be nice.