MAHOUT-1493 Port Naive Bayes to Spark DSL (for review)

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. 
in Mahout Scala, this slicing should look 

    ... weightsPerLabelAndFeature(labelIndex, ::)