参考：
http://blog.csdn.net/a819825294/article/details/51206410
http://blog.csdn.net/sb19931201/article/details/52557382
http://www.cnblogs.com/zhouxiaohui888/p/6008368.html
xgboost和GDBT的区别
https://www.zhihu.com/question/41354392
