IPv6 address to string rfc5952

Motivation:
The java implementations for Inet6Address.getHostName() do not follow the RFC 5952 (http://tools.ietf.org/html/rfc5952#section-4) for recommended string representation. This introduces inconsistencies when integrating with other technologies that do follow the RFC.

Modifications:
-NetUtil.java to have another public static method to convert InetAddress to string. Inet4Address will use the java InetAddress.getHostAddress() implementation and there will be new code to implement the RFC 5952 IPV6 string conversion.
-New unit tests to test the new method

Result:
Netty provides a RFC 5952 compliant string conversion method for IPV6 addresses
I think it would be nice to add some comments above the bit-shifting stuff to make it easier to understand what you are doing here when look at it again in 1 year or so ;)