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'd tried changing RDDFunctions but this requires changes in the java code and its related tests which just made me sad :( But propagating overloaded methods just because it is in RDDFunctions isn't great either.