Add support for magnet

To download a magnet, we don't fill the TorrentSession completely, only
with the InfoHash. We then add a few booleans for selecting the path of
a peerMessage: as long as the metadata is not completely downloaded,
we only treat metadata-oriented messages.
Merge branch 'magnet' of git://github.com/rakoo/Taipei-Torrent into rakoo-magnet

Conflicts:
	torrent.go