PERF/ENH: Use xlsxwriter by default (and then fall back on openpyxl).
Since xlsxwriter has a smaller install base (because it's newer),
reasonable to assume that if you have it installed, you want xlsxwriter.
Plus, it has better performance characteristics in general (esp. in
terms of memory usage)

Any objections to this? If you don't have xlsxwriter installed doesn't matter anyways. Going to merge soon...
