Remove O(n^2) from GBM

Was scoring all trees after each new tree was built.
Now incrementally adding the new tree to the prior scores.
Clean up column layout between GBM & DRF; document it; write accessors
&asserts to enforce consistent layout.
BulkScore takes in a column of prior tree-sums, and it adds the new tree
to prior sum.
Merge branch 'master' of github.com:0xdata/h2o