SPARK-4111 [MLlib] add regression metrics

Add RegressionMetrics.scala as regression metrics used for evaluation and corresponding test case RegressionMetricsSuite.scala.
To be consistent with other evaluation metrics, let's put the prediction as the first column. The word `value` is vague. We can use `predictionAndObservations` instead. Note that there no `s` after `prediction` to indicator that this is an RDD of pairs instead of a pair of RDDs.