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

Adding two classes: 
1) MulticlassMetrics implements various multiclass evaluation metrics
2) MulticlassMetricsSuite implements unit tests for MulticlassMetrics
This looks better to me:

~~~
weightedRecall: Double = labelCountByClass.map { case (category, count) =>
    recall(category) * count / labelCount
  }.sum
~~~