Score based on total variance (one pass only)

Be explicit about multiple scoring methods.
Also, tighten min/max.
Add lots of comments.
FluidVec parser fixes for big files

Parse last 32k buffer from large zip files.  Cache final short-reads, so
that a re-read gets the same short read.  Skip 1st line of 2nd & later
chunks in distrubuted non-zip parses.
New tests that read several files in FVecs and ValueArray and compares
them.
Distributed exceptions pass assertion errors along.
Add toString & byteSize printers to Frame, Vec & Chunk.
Add row-pretty-printer to Frame & ValueArray.
GBM: pull out the error calc into a MRTask2.
Improve several asserts.
Merge branch 'master' of https://github.com/0xdata/h2o