[ADD] Add support for Excel 2007+ XLSX format.

This PR extends the existing Excel XLS reporting based on xlwt
with support for Excel XLSX using xlsxwriter.

This allow a larger number of columns and rows and potential aceess
to the wider range of features supported by xlsxwriter:
http://xlsxwriter.readthedocs.org/index.html

This is in response to feature request: #2848
Yes. The worksheet name is still limited to 31 characters. Note, that line should actually be `add_worksheet()`.  I'll update the PR.