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

Adding two classes: 
1) MulticlassMetrics implements various multiclass evaluation metrics
2) MulticlassMetricsSuite implements unit tests for MulticlassMetrics
I see why this is necessary now. Could you move this import above spark imports? Usually we have four import groups:

1. java
2. scala
3. 3rd-party
4. spark