Adds automatic doc generation for entire project

Generate docs into `target/site` with `sbt unidoc` and publish to http://twitter.github.com/algebird with `sbt ghpages-push-site`.

If you guys like this, we can get this into scalding, storehaus, summingbird and bijection. I'd rather duplicate these few files than spend more hours fucking with SBT's plugin architecture to bundle them up separately :)