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. 
Thanks again for looking at this, Anand - I'm getting the following with patch applied and the h2o tests enabled.  Appreciate it! 

- NB Aggregator *** FAILED ***
  java.lang.IllegalArgumentException: Not a String
  at water.fvec.Chunk.set_impl(Chunk.java:189)
  at water.fvec.Chunk.set0(Chunk.java:158)
  at water.fvec.Chunk.set(Chunk.java:105)
  at org.apache.mahout.h2obindings.H2OHelper.drmFromMatrix(H2OHelper.java:334)
  at org.apache.mahout.h2obindings.H2OEngine$.drmParallelizeWithRowLabels(H2OEngine.scala:83)
  at org.apache.mahout.math.drm.package$.drmParallelizeWithRowLabels(package.scala:67)
  at org.apache.mahout.classifier.naivebayes.NBTestBase$$anonfun$2.apply$mcV$sp(NBTestBase.scala:90)
  at org.apache.mahout.classifier.naivebayes.NBTestBase$$anonfun$2.apply(NBTestBase.scala:69)
  at org.apache.mahout.classifier.naivebayes.NBTestBase$$anonfun$2.apply(NBTestBase.scala:69)
  at org.scalatest.Transformer$$anonfun$apply$1.apply(Transformer.scala:22)
  ...
- Model DFS Serialization *** FAILED ***
  java.lang.IllegalArgumentException: Not a String
  at water.fvec.Chunk.set_impl(Chunk.java:189)
  at water.fvec.Chunk.set0(Chunk.java:158)
  at water.fvec.Chunk.set(Chunk.java:105)
  at org.apache.mahout.h2obindings.H2OHelper.drmFromMatrix(H2OHelper.java:334)
  at org.apache.mahout.h2obindings.H2OEngine$.drmParallelizeWithRowLabels(H2OEngine.scala:83)
  at org.apache.mahout.math.drm.package$.drmParallelizeWithRowLabels(package.scala:67)
  at org.apache.mahout.classifier.naivebayes.NBModel.dfsWrite(NBModel.scala:144)
  at org.apache.mahout.classifier.naivebayes.NBTestBase$$anonfun$3.apply$mcV$sp(NBTestBase.scala:142)
  at org.apache.mahout.classifier.naivebayes.NBTestBase$$anonfun$3.apply(NBTestBase.scala:117)
  at org.apache.mahout.classifier.naivebayes.NBTestBase$$anonfun$3.apply(NBTestBase.scala:117)
  ...