Add extraction for release dates for the new-format (at least) ninjatune xls files

Mostly putting up for review for the sake of this being a way to have people able to question my stuff and learn about the codebase.

This parses out the release dates in the newer of the two ninjatune xls formats (the old one has strings, as I recall, and I'm not sure it splits by country -- I haven't checked yet).