Added Windows version support (XP, Vista, 7, 8)

I added support (and tests) to tell if a browser is being run on Windows XP (32 & 64-bit), Vista, 7 or 8. Since we wanted to detect if a user was using Windows XP, it would just be browser.windowsXP? to provide more differentiation outside of just browser.windows?. While this feature could exist for other platforms, I think this is probably the most useful since there are so many more versions of Windows than any other platform (kind of the same reason it is more handy to have individual methods for different versions of IE than for Chrome, Firefox, etc.).

Note: this branch also merges the previous pull request for IE10 (https://github.com/fnando/browser/pull/31), since I needed that as well.