Implemented WebSeeds and Added Backup trackers. Removed transmissionrpc 

Upgraded from the original Bittorrent implementation to the most recent
I could find:
http://sourceforge.net/projects/pingpong-abc/files/ABC-source/3.1/
Stripped all irrelevant files (GUI, etc.)
Modified `BitTornadoABC.make_meta_file` so that it accepts 'url-list'
as a key in params and dumps it to .torrent-files.

(see http://www.bittorrent.org/beps/bep_0019.html#metadata-extension)

Added BITTORRENT_TRACKER_BACKUP (comma-separated list), beause `tracker.ccc.de` is down at the moment.
Removed obsolete transmissionrpc.