Add some logic to buffer log messages at startup that otherwise would
be dropped.

This was annoying because version info was getting dropped, and we
definitely want that if customers report issues.
Merge branch 'master' of github.com:0xdata/h2o
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