101 Fix scaladoc warnings - fixed in streaming but leaving the rest as i...

...s. This is a known bug that will be fixed in a later version of Scala. Minor cleanup of build and addition of sbt unidoc plugin
I am wondering if we shouldn't make it `Option[Seq[String]]` instead. IMHO it will be more consistent and more predicable - for example, when the use accidentally pass an empty sequence, he would be informed with the appropriate exception instead of returning all columns.