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).
The named function idea might be good. I tried to turn all the country ones into a list comprehension, but it turns out python does stupid things with closure scope, heh. Specifically, a lambda that references the list comprehension's loop variable will only ever see the last value in the list comprehension, so it was trying to put everything in LU rather than their own respective countries.

Removing the lambda for the path might actually remove that problem, though.