version bump 0.7.4: I/O improvements

- basic style + theme parsing, option .cellStyles (h/t @eladxxx)
- more XLSB writing stubs
- correct resolution of .xml/.bin files
- sheet_to_json improvements from js-xls
o opts.header = 1 for array of arrays
o opts.header = 'A' for spreadsheet column labels
o custom opts.header array for custom labels
o opts.range = n starts from row n
o opts.range = range restricts writer to work within the specified range
- Makefile adapted to work with cygwin on windows