[MLLIB] SPARK-4362: Added classProbabilities method for Naive Bayes

Added methods which accept an RDD or array and return a map of (label -> posterior prob.) for each input set rather than only returning the key with the maximum value. 
This can fit on 1 line.
Also, there's no need to specify the type of probs since mutable.Map.empty[Double, Double] makes it clear.