Implemented WebSeeds and added backup trackers. Removed transmissionrpc 

Rerequest Pull #13. Chunked it into smaller commits makes it easier to follow.

Upgraded from the original Bittorrent/BitTornado 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 into .torrent-files. (see http://www.bittorrent.org/beps/bep_0019.html#metadata-extension)
That makes `transmissionrpc` obsolete.

Added `BITTORRENT_TRACKER_BACKUP` (comma-separated list), beause tracker.ccc.de is down at the moment.

To upgrade to Webseeds. recreate your .torrent files:
```bash
$ python manage.py shell
```
```python 
>>> from videoportal.models import Video
>>> for v in Video.objects.all():
>>> ...    v.create_bittorrent()
>>>
```