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
Ok, but this requires deeper changes, because:
- it should work consistently, no matter what combination of arguments is passed to the method (this should be changed probably on the lower level)
- it should be changed also in `RDDFunctions`
- it should follow update of the docs