Bugfix.

Parsing the same file twice lead to NPE. The newly parsed array let chunks were removed from KV when replacing old array let header by the new one. (UKV.remove removes all chunks of an array let and the new and old chunks had the same keys).

DParseTask now removes old VA before launching pass 2.
Merge branch 'master' of github.com:0xdata/h2o
Change default name of an re-uploaded files.  In case there is already a key with the same name (dataset.hex), default name would include number so that the new file is unique (e.g. dataset1.hex, dataset2.hex,...).
Fix of bad assumption that arrays with trees' errors are of the same
size.
Assertion refined (for EC2 testing)
Merge branch 'master' of github.com:0xdata/h2o
Fix to get parsing of .xls work with the new Parse UI. Currently .xls files recognized only by suffix.
Merge branch 'master' of github.com:0xdata/h2o
Explicit header option for parser.
Merge branch 'master' of github.com:0xdata/h2o
Remove double-assign
Include stack dump from F/J tasks
Toss out columns which are >20% NA's.

Helps most algo's which don't handle mostly-NA-cols by default.
Also, in pass2 parsing if the col is declared a STRINGCOL then
do not parse it as a number (then fail back to string parsing).
Just start out with string parsing.  In the matching tests, increase
the dataset sizes to prevent NA'ing whole columns.
Replaced column names by numbers in HexColumnSelect (only affects returned URL, still displayed as names in the multiselect)..
Merge branch 'master' of github.com:0xdata/h2o
hooks to pass and do arbitrary alternate json [e.g. noise=("StoreView",None)] request during poll_url
don't do x=9 because GLM stack traces if you give it an x=<col> where <col> was dropped because it was a constant column in the browser. I guess this might mean that GLM can't accept constant columns?
back off on lambda=1 on some GLMs
increase some multijvm timeouts for EC2
fix typo in verboseprint noise/parse in h2o.py
typo. was 'args' needed to be 'kwargs'
typo on param for 'noise'