[MLLIB] [SPARK-2222] Add multiclass evaluation metrics

Adding two classes: 
1) MulticlassMetrics implements various multiclass evaluation metrics
2) MulticlassMetricsSuite implements unit tests for MulticlassMetrics
RDD functions return "scala.collection.Map" in various methods that I use: countByValue, collectAsMap etc. Returning scala.collection.Map is not consistent with Scala predef for Map which is "collection.immutable.Map" and the compiler returns type mismatch error if I remove this import.