Support openpyxl

As xlwt generated spreadsheets crash my latest Excel 2013, I hacked a
quick support for openpyxl, which, to my deepest suprise, works. I had
to modify test case to support it and I also hacked the
get_excel_support function to be able to dynamically (during runtime)
get Excel support settings (module-level defined constants make testing
harder). I'd love to hear your feedback on this patch, if it is good,
please feel free to include it in the main branch.