New plug-in: Windows Live Tiles

Huge README file explaining most of it.

The whole thing is done in such a weird way from Microsoft’s side (why periodically request five separate XML files instead of just one?) so I’m okay with doing the XML generation in a weird way as well.

P.S: I’d like to do a blog post about it on the Nikola blog when this hits the plugin repo’. :wink:

cc/ @Kwpolska.
Drop the `generate_` here, too.