Implementation Process - xgboost methods

[antoinecarme]

At least gbTree method

for multiclass classification and regression

As described in issue #2

references :

https://cran.r-project.org/web/packages/xgboost/index.html
https://cran.r-project.org/web/packages/xgboost/xgboost.pdf
https://github.com/topepo/caret/blob/master/models/files/xgbTree.R
https://github.com/cran/xgboost
https://github.com/dmlc/xgboost

[antoinecarme]

Added xgboost classification and regression models support