Human-Readable Names Resolution

As [Human-Readable Names](https://github.com/bitcoin/bips/pull/1551) start getting more traction, having common Bitcoin libraries support the requisite DNS lookups, DNSSEC validation, and BIP 21 parsing would go a long ways towards simplifying wallets. The only actually hard part of all this is DNSSEC validation, which can be accomplished either through https://git.bitcoin.ninja/index.cgi?p=dnssec-prover or if you trust the DNS resolver and check the Authenticated Data bit in the response.