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

Adding two classes: 
1) MulticlassMetrics implements various multiclass evaluation metrics
2) MulticlassMetricsSuite implements unit tests for MulticlassMetrics
Please follow [Spark Code Style Guide](https://cwiki.apache.org/confluence/display/SPARK/Spark+Code+Style+Guide) and code style used in the code base.

~~~
.map { case (prediction, label) =>
  (label, if (label == prediction) 1 else 0)
}.reduceByKey(_ + _)
.collectAsMap()
~~~

1) new line after " =>"
2) space after "if"
3) change "{ }" to "( )"
4) use "()" for an action