Update project to build against http4s-0.2.0

Thanks for your interest in http4s! We were interested in building a httpbin clone, but it appears you've already done it, and done it pretty well.
We are on 0.3.0-SNAPSHOT now, so the project didn't build. I pinned it to the stable 0.2.0 artifacts in Central and updated the things that had changed during development.
Please let me know if I messed anything up.