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

Adding two classes: 
1) MulticlassMetrics implements various multiclass evaluation metrics
2) MulticlassMetricsSuite implements unit tests for MulticlassMetrics
It is not necessary to generate two Arrays. `for` in Scala is also slow. Please allocate an `Array[Double]` of size `n * n` and assign the elements directly.