Added Worksheet.use_start_page_number.

Currently, xlwt hardcodes the Worksheet is hardcoded to always use the Worksheet.start_page_number for the first page of the worksheet when &P is in the header or footer.

If you are printing multiple sheets at once, it is nice to have automatic page numbering, so each successive worksheet starts where the last one left off. It is especially confusing since &N in the header or footer will display the total number of pages for all the sheets selected for printing. This can yield three worksheets being printed out with the header "1 of 3", "1 of 3", "1 of 3" instead of "1 of 3", "2 of 3", "3 of 3".

This patch simply adds a new property to override the value in the BIFFRecord.