Added a Changelog page

Which fetches data from the NEWS file in Picard's repo and builds an html version of it.

Live at: http://musicbrainz.github.io/picard-website/changelog.html
In Chromium I see this request getting sent with no-cache headers, which is odd since I think $.ajax defaults to caching things, though maybe it's getting overridden somewhere that you know of and has to be explicitly set. But yeah, the file isn't ginormous but it makes sense to have the browser cache it given how infrequently it changes.