Add optional timeout to get PROXY header

The library user can define a maximum time to wait
for the PROXY protocol header, before failing out to
normal connection.

We can assume that a proxy in front of the service will
send the PROXY header immediatelly.

This solves the issue of clients getting block when
getting the RemoteAddr() for an incoming connection that
does not send any data. That is the case of http.Serve on
go < 1.6 as described in https://github.com/armon/go-proxyproto/issues/1

README.md

@@ -28,7 +28,7 @@ Using the library is very simple:
 list, err := net.Listen("tcp", "...")
 
 // Wrap listener in a proxyproto listener
-proxyList := &proxyproto.Listener{list}
+proxyList := &proxyproto.Listener{Listener: list}
 conn, err :=proxyList.Accept()
 
 ...