DNS codec for Netty which is based on the work of [#1622].

Motivation:
As part of GSOC 2013 we had @mbakkar working on a DNS codec but did not integrate it yet as it needs some cleanup. This commit is based on @mbakkar's work and provide the codec for DNS.

Modifications:
Add DNS codec

Result:
Reusable DNS codec will be included in netty.

This PR also includes a AsynchronousDnsResolver which allows to resolve DNS entries in a non blocking way by make use
of the dns codec and netty transport itself.