Implement BEP 12 Multitracker Metadata Extension

See http://www.bittorrent.org/beps/bep_0012.html
Support IPV6 trackers.

Implemented BEP 7 "IPv6 Tracker Extension"

Tested with http://ipv6.torrent.ubuntu.com/

To test IPv6 using the ubuntu torrents I had to manually modify
startTrackerClient() to skip the first level of the announce list. The
Ubuntu torrents use an IPv4 tracker as the first level tracker, and
only use ipv6.torrent.ubuntu.com as the second level tracker.
Implement lpd

As should be defined in BEP-0014 (still TBD)

Basically, for each torrent we are in, a HTTP-like message is sent to
239.192.152.143:6771. This message is an announcement that we are in
the swarm.

When a Taipei-Torrent peer receives such a message, it will
automatically connect unless it's already connected.

Tested successfully with aria2c and transmission.
Fix multiple tidbits

- Remove infohash from activeAnnounces map when we stop it
- Move useLPD flag to lpd.go and set it to false by default
- Really set the ticker to 5 minutes
- Use a default value for the lpd struct in main.go so we can use
  lpd.announces in the select loop
Merge pull request #39 from rakoo/add-lpd

Implement lpd