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

Adding two classes: 
1) MulticlassMetrics implements various multiclass evaluation metrics
2) MulticlassMetricsSuite implements unit tests for MulticlassMetrics
Save `labels.size` to `n`? Btw, I'm not sure whether we should use `lazy val` here because the result matrix could be 1000x1000, different from other lazy vals used here.