Added support for the new Windows Firefox UA style

I recently created [browser_details](https://github.com/gshutler/browser_details) and I've seen a few browsers come through for Firefox 17 that got classified as:

    Firefox 17.0 (Windows NT 6.1, WOW64)

Whereas I would expect these to be reported as:

    Firefox 17.0 (Windows, Windows 7)

From what I can find, Firefox recently changed its UA format. I found some examples here http://www.useragentstring.com/pages/Firefox/ that I based my tests on.

There's no localization specified in this UA so I've explicitly check for that. Let me know if you want things tidying up. It felt a bit thrown together but I thought a PR was better than an issue.