Refactor client.go.

- Add WantUnicastResponse to allow sending mDNS queries that desire unicast
  responses (see section 5.4 in RFC6762).
- Listen for UDP packets sent to the mDNS multicast addresses ("ipv4:
  224.0.0.251", ipv6: "ff02::fb")
- Fix bug on line 177 of client.go: Ignored error during sendQuery.
- Add TODO comments regarding unimplemented features of mDNS protocol.
- Add error logging statement when DNS packet parsing failed.