Add HAProxy 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.
Move haproxy codec to a separate module
Overall refactoring of the haproxy codec

- Convert constant classes to enum
- Rename HAProxyProtocolMessage to HAProxyMessage for simpilicity
- Rename HAProxyProtocolDecoder to HAProxyMessageDecoder
- Rename HAProxyProtocolCommand to HAProxyCommand
- Merge ProxiedProtocolAndFamity, ProxiedAddressFamily, and
  ProxiedTransportProtocol into HAProxiProxiedProtocol and its inner
  enums
- Overall clean-up