xgboost原理

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