Bug w/C2SChunk not having correct bias
Implement C2Chunk inflation
Correct length of C1 & C1NChunk
Implement C2SChunk set8
Actually build & run distributed trees.

Last non-distributed data pass gets distributed.
Histograms get a 'reduce' and do proper array alloc in the map call.
Run & fix a few bugs with prior MRTask2 passes.
Compute regression (squared error) & classification errors.
Turn off some debug printing.  Clean up treeMin/treeMax.
Remove unneeded (leaked) arrays as tree building progresses.
Merge branch 'master' of https://github.com/0xdata/h2o