[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:

• [x] build_forest #214 @ablaom
• [x] apply_forest
• [x] apply_forest_proba
• [ ] build_tree (keep #224 in mind)
• [ ] print_tree
• [ ] apply_tree
• [x] apply_tree_proba
• [ ] build_adaboost_stumps
• [ ] apply_adaboost_stumps
• [x] apply_adaboost_stumps_proba
• [x] 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.