Stepwise selection from the full model and use direction = "both"
direction = "both" is too slow. use default as "backwards"
Include over dispersion parameter later
Cross validation for gbm
gbm added. But it didn't do better than others
Nice results

> print(round(score,4))
  poisReg poisReg.offset negBino negBino.offset gbmPois gbmPois.offset   tree tree.offset glmnet glmnet.offset
1  1.3389         1.3433  1.3088         1.3115  1.3612         1.3768 1.3699      1.3008 1.2876        1.2906