Add ELB proxy protocol support to the HTTP module.

Previously, this was only supported with http-socket.  If you're
using http-socket, you probably have a webserver on top of you--and if
you have that, then uWSGI isn't receiving the proxy protocol anyway.
Merge pull request #627 from blue-goji/proxy-protocol-for-http-module

Add ELB proxy protocol support to the HTTP module.