Improve date formatting in spreadsheets

[Adam Winer]

Original issue 167 created by adam_winer on 2014-05-25T05:29:19.000Z:

http://poi.apache.org/spreadsheet/quick-guide.html#CreateDateCells

... though this may not be practical, given Scythebill's support for partial dates.

[Adam Winer]

c7dacc9 tries this out.