Created python/pyspark/mllib/data.py for DatasetInfo.  Added Python DecisionTree print() functions.
Small Python API updates: Using py4j for converting Python dict to Java Map, for categoricalFeaturesInfo.  Added trainDecisionTreeRegressorModel() to PythonMLLibAPI.
Updates for Python decision tree API:
* Added tree.py example
* Renamed DecisionTree model and node print() functions to explain().
* Updated usage doc in tree.py for DecisionTreeClassifierModel.
* Added defaultParams() and train() static functions for Python DecisionTreeClassifier/Regressor.