Add the magnet links support

Major changes: 
- Downgraded cowboy (because bullet);
- Added cascadae as a dependency (to start it, use `cascadae:start()` from console);
- Added console logging for torrents as in Transmission;
- Fixed common tests, moved into this repository.

What does not work: 
- proper tests (from DHT);
- mocked eunit-tests (from dotdir).

What was added:
- `bep9` common test;
- metadata downloader (see `etorrent_magnet`);
- `etorrent_ext` is a wrapper, that hides extended message ids.

TODO:
- downloading metadata into a torrent file (using `etorrent_dotdir` as a place to save this file?), starting the file.

Testing:
- Install `transmission-cli`, `opentracker`.
- Run `./ct-run.sh`.