Upstream Proxy Protocol support

[chendo]

We'd like the upstream servers to be notified of the original IP and port that the client connection is coming from. Currently, we're using HAProxy and its accept-proxy v1 support.

The spec is defined here: http://www.haproxy.org/download/1.5/doc/proxy-protocol.txt

nginx already supports proxy_protocol inside the listen directive, but that is only for if something else is proxying in front of nginx.

[chendo]

I've hacked together this feature in #110, but it needs significant cleanup.