[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. 
Scala newbie. I couldn't find a better pattern to build the map than mutating it in the foreach. Should I just build a map then make it immutable for returning?