Update dependency on bittorrent-tracker

Fixes handling big files
https://github.com/feross/bittorrent-tracker/issues/11
Merge pull request #21 from thermatk/patch-1

Update dependency on bittorrent-tracker
override DHT size

using DHT to find peers can be a resource-intensive operation - allow
users to override the defaults
Merge pull request #22 from asapach/dht

override DHT size
bump tracker to 0.4.1
0.7.0
bump dht to 0.7.0
0.7.1
Merge pull request #2 from mafintosh/master

sync