expose H2OGridSearch models

Expose access to individual GridSearch models.

It would be also nice to be able to sort the models according to some metric similar as in h2o such as:

`fitted_grid.get_grid(sort_by='rmse', decreasing=False)`

Or at least provide a DataFrame, showing some metrics such as ACCURACY/AUC/F1 for classification, RMSE/MAE for regression of individual models.