Add travis.ci support

This makes it so travis.ci builds optar. It builds it both ways, with gcc and with clang, which seems pretty neat to me.

This will need one last tweak to the image link to point to your repo and branch instead of mine, but that has to wait until you've enabled it over on travis.

I've also Markdown-ified the readme, since it was already almost there.

I recognize these changes are kind of superfluous, so if you don't take these changes, you won't hurt my feelings. :)