ENH: Adding XlsxWriter as an ExcelWriter() option
related/closes #4517

Hi,

I am the author of the [XlsxWriter](https://github.com/jmcnamara/XlsxWriter) module, a Python module for writing Excel XLSX files.

I am interested in submitting a PR to add this as a option when writing Excel xlsx files via `to_excel()`.

It has a similar interface to the xlwt module but creates xlsx files. For large data files it is ~10x faster than OpenPyXL while maintaining low and constant memory usage. I occasionally see the current xlsx writer's speed and memory usage mentioned as a bug/limitation so it could be useful to have a fallback module option in those cases.

XlsxWriter also supports features such as charts and auto-filters that could be exposed at a later stage. See https://xlsxwriter.readthedocs.org/ for more details.

It uses standard libraries only and has an extensive test suite for Python 2.6, 2.7, 3.1, 3.2 and 3.3.

If you are interested I can work on a prototype and send a PR for you to evaluate. I have read CONTRIBUTING.md and I think I should be able to meet the criteria.

Regards,

John.
