client IPv6 support [BEP-7]

Depends on the next versions of the following modules:
* string2compat
* compat2string
* addr-to-ip-port

You can test with torrents from Bitlove.org, the tracker always returns the IPv4 & IPv6 address of the prittorrent seeder.
Once we support ipv6 on the server (https://github.com/feross/bittorrent-tracker/issues/43) we can easily add a test that tests both sides of this. :)