rkt: fetch should support bittorrent via magnet URIs

[jonboulle]

When performing image meta discovery, or run directly with a magnet URI on the command line (e.g. rkt fetch magnet:?xt=urn:btih:TDC4GYOQXZPSUB7KR6SQKLS2USAJPZ7Z&tr=tracker.example.com%3A80%2Fannounce) rocket should be able to retrieve images via the BitTorrent protocol.

Specifically, it should support magnet URIs that point to a combination of address tracker and BitTorrent info hash

In principle the magnet support can be extended further in future (e.g. falling back using the "acceptable source" parameter, or other protocols), but I suggest we start with BitTorrent and take it from there.

[jackpal]

FWIW I wrote a BSD-licensed golang torrent library here: https://github.com/jackpal/Taipei-Torrent
It has support for magnet links courtesy of https://github.com/nictuku/dht

You're welcome to use it or just look at it for parts and ideas. I made it modular enough to allow self-contained tests, so I think that it would be possible to use those same APIs to reuse the code in your project.

I guess a potential problem is that the torrent protocol has a large number of parameters that users might need to set to allow the torrent to work for their network configuration. You'd have to figure out how to allow the user to set those parameters.