Added ability to take header form another key loaded in h2o during parse.

User can pick header as existing H2o Key.

The key with headers can be either:
   1) one of the files being parsed, in this case, if the file contains both header and data, it will determine the GLOBAL SETUP (i.e. all other files must have the same separator and matching number of columns and of course the same file type ). of the parse.
   2) raw file with single line containing header with correct number of columns  and NO DATA
   3) ValueArray or Frame with correct number  of columns
Merge branch 'master' of github.com:0xdata/h2o
Minor bug fix in picking header file. MAde the field non-mandatory.
Minor bug fix in header selection, Set header checkbox properly.