Spent a couple days adding a [[bittorrent_special_remote|special_remotes/bittorrent]]
to git-annex. This is better than the demo external torrent remote I made
on Friday: It's built into git-annex; it supports magnet links; it even
parses aria2c's output so the webapp can display progress bars.

Besides needing `aria2` to download torrents, it also currently depends on
the `btshowmetainfo` command from the original bittorrent client (or
bittornado). I looked into using
<http://hackage.haskell.org/package/torrent> instead,
but that package is out of date and doesn't currently build. I've got a
patch fixing that, but am waiting to hear back from the library's author.

There is a bit of a behavior change here; while before `git annex addurl` of
a torrent file would add the torrent file itself to the repository, it now will
download and add the contents of the torrent. I think/hope this behavior
change is ok..
