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`error: 'exp' is not a member of 'std'
     rec[i] = std::exp(rec[i] - wmax);`

Support multiclass classification (#53)

To solve #41

add feature importances (#42)

update for typo.

# Conflicts:
#	src/io/dataset.cpp
#	src/io/ordered_sparse_bin.hpp
#	src/treelearner/leaf_splits.hpp
#	src/treelearner/serial_tree_learner.cpp

merge to master.

support max_depth option, to solve (#33)