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
SslHander wrap conditional direct buffer allocation

Motivation:
The SslHandler currently forces the use of a direct buffer for the input to the SSLEngine.wrap(..) operation. This allocation may not always be desired and should be conditionally done.

Modifications:
- Use the pre-existing wantsDirectBuffer variable as the condition to do the conversion.

Result:
- An allocation of a direct byte buffer and a copy of data is now not required for every SslHandler wrap operation.
Backport header improvements from 5.0

Motivation:
The header class hierarchy and algorithm was improved on the master branch for versions 5.x. These improvments should be backported to the 4.1 baseline.

Modifications:
- cherry-pick the following commits from the master branch: 2374e17, 36b4157, 222d258

Result:
Header improvements in master branch are available in 4.1 branch.