Fix prediction with binary classifier in GBM.

GBMModel did not know that a tree is composed only of one internal tree
instead of 2. Hence, GBMModel.score() function was updated.
Corresponding fix was propagated into generated model.
New R tests + minor fixes in shared R-code.

Javapredict GBM shared code now save results from h2o in
the same way as DRF shared code.