Add Transformer and Estimator for GBTClassifier and GBTRegressor

Add Transformer and Estimator for GBTClassifier and GBTRegressor.

They are supposed to cover most features in SparkML provides, except for the following features:

1. Support early-stopping with validation set.
2. Support weights.
3. Support leaf ID.
4. Support feature importance.
5. Support 1-order gradient.