Allow depth==1
Fix numerical stability on perfect prediction.

Turns out a perfect-prediction for 1 class gets a Gamma of 0/0, and a
perfect MIS-prediction gets inf/0.  Distinguish the cases.  Now seeing
iris with large learn-rate stable out to 100,000 trees.
Merge branch 'master' of github.com:0xdata/h2o
Merge branch 'master' of github.com:0xdata/h2o