Handling empty typeless cells

@nathanathan presented a case generated from excel, which had the cell:

    <c r="E1" s="1"/>

The schema is incorrect here (advising to treat it as a NaN value).  The right
thing to do is to treat it as `undefined`.
refactoring the csv generation code

XLSX.utils.sheet_to_csv
Handling Google Docs Quirks

closes #2
version bump 0.0.6