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

Adding two classes: 
1) MulticlassMetrics implements various multiclass evaluation metrics
2) MulticlassMetricsSuite implements unit tests for MulticlassMetrics
nit: It is a collection of (prediction, label) pairs. Should we call it `predictionAndLabels`? `predictionsAndLabels` sounds to me like an instance of `(RDD[Double], RDD[Double])`. This is minor, but we should be consistent across the codebase. I prefer `predictionAndLabels` and I saw you used it in the multilabel PR as well.