Don't write cols tag when it donesn't exist
Fix Fill tags
Add date1904 option for workbook

Excel for Windows uses a default epoch of 1900 and Excel
for Mac uses an epoch of 1904. However, Excel on either
platform will convert automatically between one system
and the other. QtXlsxWriter stores dates in the 1900 format
by default.
Add basic QDateTime support
Refactor: D-Pointer used for public classes