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