SPARK-4111 [MLlib] add regression metrics

Add RegressionMetrics.scala as regression metrics used for evaluation and corresponding test case RegressionMetricsSuite.scala.
The third and the forth columns are not necessary. You can use `normL1` and `normL2` on the second column:

https://github.com/apache/spark/blob/master/mllib/src/main/scala/org/apache/spark/mllib/stat/MultivariateOnlineSummarizer.scala#L219