fvec parser progress

ByteVec.openStream takes a progress key, and updates as the file is
read.
Means we can update unzipping as the raw zip data is read.
Use C1NChunk instead of C1Chunk, since no NA's in raw text files.
Peel up to 2 extensions from standard filenames (.csv.gz)
Merge branch 'master' of github.com:0xdata/h2o