Multi-tree GBM appears to be (barely) working

No learning-rate yet, so it's 100% LR per tree.
Tree limit pegged @ 20 right now.
Clean out double NA's - force NaN as the only NA.
More pretty-print logic for DTree.
Keep bottomed out rows' NID's instead of setting them to -1.
Merge branch 'master' of github.com:0xdata/h2o
Cleanup NA handling proper