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. 
Imports look engine-independent. Should not be Spark-coupled. 
Imports probably should include implicit operations per document (which is why the code does weird stuff like `new MatrixOps(m)`).

The "standard" recommended way to do imports for engine -independent code

     import org.apache.mahout.math._
     import scalabindings._
     import RLikeOps._
     import drm._
     import RLikeDrmOps._

if java collections are used (e.g. something like `for (row <- matrix) { ... }`) then it also would need 

    import collection._
    import JavaConversions._

to enable all implicits.