MAHOUT-1493 Port Naive Bayes to Scala DSL 

Created this PR to review the patch provided by Christoph Viebig and team, Students of TU, Berlin.  It is based on Sebastian's code from before the DSL was abstracted away from Spark. 
Argh, the formatting got messed up. Please replace "writer.set(r, rmap.get(r));" with "labels.chunkForRow(r).set(r, rmap.get(r));"