Modify file timestamp to create consistent binary file.
Modify file timestamp to create consistent binary file.

Modification to enable producing consistent binary output for
the same worksheet data. Uses 1980-01-01 date for file, like
Excel.

Issue  #494
jmcnamara/XlsxWriter,jmcnamara/XlsxWriter,jmcnamara/XlsxWriter,jmcnamara/XlsxWriter