Add support for bittorrent magnet URIs

For now it's just an URI endpoint, but I would like to expose it on the view, pretty much like other torrent sites do.

@ialbert, do you think we can just go ahead and give prio to magnets instead of .torrent files? Apparently it saves quite a bit of space:

http://thenextweb.com/insider/2012/01/13/the-pirate-bay-says-it-will-no-longer-serve-torrents-shifts-to-magnet-links/

I think it's better to jump in now than having to code and perform migrations later.

@nimiq, @mozillascience