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.
From my reading of the [spec](http://www.bittorrent.org/beps/bep_0007.html), the `peers6` key should only contain ipv6 addresses.

> peers6 contains address-port pairs where the addresses are all IPv6

And IPv6 addresses seem to always have `:` because that's the delimiter. So I can't figure out what this line is for.

I'll remove it for now, but let me know if it's important and we can re-add it.