[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
this is mostly correct.  It will not work, but it's close.  What your change does is display/hide the xls export button depending on the result of the xlsx check.  But you did not add an xlsx button, so there is actually no way that the export_xlsx_data case below will be activated.  To fix this, here is what needs to be done:
1. add an export to xlsx button next to the export xls button (in web_graph.xml) with a data-choice attribute of export_xlsx_data
2. change the code above to toggle the new button
3. in option_selection,