[SPARK-5496][MLLIB] Allow both classification and Classification in Algo for trees.

[mengxr]

to be backward compatible.

[SparkQA]

Test build #26393 has started for PR 4287 at commit a025c53.

• This patch merges cleanly.

[SparkQA]

Test build #26393 has finished for PR 4287 at commit a025c53.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • class TreeEnsembleModel(JavaModelWrapper):
  • class DecisionTreeModel(JavaModelWrapper):
  • class RandomForestModel(TreeEnsembleModel):
  • class GradientBoostedTreesModel(TreeEnsembleModel):
  • class GradientBoostedTrees(object):

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/26393/
Test PASSed.

[mengxr]

Merged into master.