Some more work on the interface that lets remotes claim urls for `git annex
addurl`. Added support for remotes suggesting a filename to use when
adding an url. Also, added support for urls that result in multiple files
when downloaded. The obvious use case for that is an url to a torrent that
contains multiple files.

Then, got `git annex importfeed` to also check if a remote claims an url.

Finally, I put together a quick demo external remote using this new
interface. [[special_remotes/external/git-annex-remote-torrent]]
adds support for torrent files to git-annex, using [aria2c](http://aria2.sourceforge.net/) to download them.
It supports multi-file torrents, but not magnet links. (I'll probably
rewrite this more robustly and efficiently in haskell sometime soon.)

Here's a demo:

<pre>
# git annex initremote torrent type=external encryption=none externaltype=torrent
initremote torrent ok
(Recording state in git...)
# ls
# git annex addurl  --fast file:///home/joey/my.torrent
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   198  100   198    0     0  3946k      0 --:--:-- --:--:-- --:--:-- 3946k
addurl _home_joey_my.torrent/bar (using torrent) ok
addurl _home_joey_my.torrent/baz (using torrent) ok
addurl _home_joey_my.torrent/foo (using torrent) ok
(Recording state in git...)
# ls _home_joey_my.torrent/
bar@  baz@  foo@
# git annex get _home_joey_my.torrent/baz
get _home_joey_my.torrent/baz (from torrent...) 
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:-100   198  100   198    0     0  3580k      0 --:--:-- --:--:-- --:--:-- 3580k

12/11 18:14:56 [NOTICE] IPv4 DHT: listening on UDP port 6946

12/11 18:14:56 [NOTICE] IPv4 BitTorrent: listening on TCP port 6961

12/11 18:14:56 [NOTICE] IPv6 BitTorrent: listening on TCP port 6961

12/11 18:14:56 [NOTICE] Seeding is over.
12/11 18:14:57 [NOTICE] Download complete: /home/joey/tmp/tmp.Le89hJSXyh/tor

12/11 18:14:57 [NOTICE] Your share ratio was 0.0, uploaded/downloaded=0B/0B
                                                                               
Download Results:
gid   |stat|avg speed  |path/URI
======+====+===========+=======================================================
71f6b6|OK  |       0B/s|/home/joey/tmp/tmp.Le89hJSXyh/tor/baz

Status Legend:
(OK):download completed.
ok                      
(Recording state in git...)
# git annex find
_home_joey_my.torrent/baz
# git annex whereis _home_joey_my.torrent/baz
whereis _home_joey_my.torrent/baz (2 copies) 
  	1878241d-ee49-446d-8cce-041c46442d94 -- [torrent]
   	52412020-2bb3-4aa4-ae16-0da22ba48875 -- joey@darkstar:~/tmp/repo [here]

  torrent: file:///home/joey/my.torrent#2
ok
</pre>