Added support for multi-file torrents. Fixed issues.

[ydavs]

Added support for multi-file torrents.

Fixed bugs:

1. I saw message length is not consumed in the case of keep-alive messages, which leads to an infinite loop when a keep-alive message is received. Fixed that.
2. Sending out bitfield is not mandatory. Some trackers don't send out bitfields but instead rely on Have messages. Added support for such a tracker.
3. Storing pending requests is implemented as an immutable object(namedTuple). This raises an error when a re-request is issued. Instead changed it to use a dictionary.

[eliasson]

Wow, I was not expecting any PR on this old project, thanks @ydavs ! =)

I'm just going to try it out before merging, hopefully in the next few days.