[Tracking Issue] Add document strings to public methods

[ablaom]

Here's what I think are intended to be public, ranked in rough order of priority:

• build_forest Add docstring for build_forest #214 @ablaom
• apply_forest
• apply_forest_proba
• build_tree (keep documentation: Clarify n_subfeatures in build_tree? #224 in mind)
• print_tree
• apply_tree
• apply_tree_proba
• build_adaboost_stumps
• apply_adaboost_stumps
• apply_adaboost_stumps_proba
• prune_tree
• nfoldCV_forest
• nfoldCV_tree
• nfoldCV_stumps
• load_data
• DecisionTree.confusion_matrix
• DecisionTree.util.gini
• DecisionTree.util.entropy
• DecisionTree.util.zero_one
• impurity_importance
• split_importance
• permutation_importance

Any help here appreciated. Take a look at #214 to get an idea of existing format.