DecisionTree tests for spark-perf

MLAlgorithmTests.scala
* DecisionTreeTests: abstract class extending RegressionAndClassificationTests, to be combined after the MLlib API update.
* DecisionTreeTest

DataGenerator.scala
* generateDecisionTreeLabeledPoints, with helper methods:
** randomBalancedDecisionTree
** randomBalancedDecisionTreeHelper
** ClassLabelPairGenerator
** RealLabelPairGenerator
** FeaturesGenerator