Add proxy protocol decoder

Motivation:
The proxy protocol provides client connection information for proxied network services. Several implementations exist (e.g. Haproxy, Stunnel, Stud, Postfix) but the primary motivation for this implementation is to support the proxy protocol feature of Amazon Web Services Elastic Load Balancing.

Modifications:
This commit adds a proxy protocol decoder for proxy protocol version 1 as specified at http://haproxy.1wt.eu/download/1.5/doc/proxy-protocol.txt. The foundation for version 2 support is also in this commit but it is explicitly NOT supported due to a lack of external implementations to test against.

Result:
The proxy protocol decoder can be used to send client connection information to inbound handlers in a channel pipeline from services which support the proxy protocol.