Add reverse proxy endpoint.

It works by using the Host of the request. When it receives something in
the form of: `<node>.<session>.play-with-docker.com` it does a reverse
proxy http request to `node`, validating that the `node` actually belongs
to the `session`.
If the node has a prefix `ip` and continues with a valid IP address
where the dots where replaces by underscores (like `ip10_0_0_1`) then it
will remove the `ip` prefix and and replace the underscores by dots, and
assume it is an ip address.

api.go

@@ -59,6 +59,9 @@ func main() {
 
 	r.Handle("/sessions/{sessionId}/ws/", server)
 
+	// Reverse proxy
+	r.Host(`{node}.{session}.play-with-docker.com`).Handler(handlers.NewMultipleHostReverseProxy())
+
 	n := negroni.Classic()
 	n.UseHandler(r)