LightGBM

add predict_leaf_index option (#30)

* a python regression example

* add early-stopping

* fix bugs

* remove not needed files

* update easy-stopping; fix warnings; add alias

* add comments; remove useless line

* change code order

* change Print to PrintAndGetLoss: return loss

* change comment; add bracket

* change return type of PredictLeafIndex in boosting.h

* move string cast into lambda function

* move if-then outside predict_fun