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.

handlers/reverseproxy.go

@@ -0,0 +1,37 @@
+package handlers
+
+import (
+	"net"
+	"net/http"
+	"net/http/httputil"
+	"strings"
+
+	"github.com/gorilla/mux"
+)
+
+func NewMultipleHostReverseProxy() *httputil.ReverseProxy {
+	director := func(req *http.Request) {
+		v := mux.Vars(req)
+		node := v["node"]
+		if strings.HasPrefix(node, "ip") {
+			// Node is actually an ip, need to convert underscores by dots.
+			ip := strings.Replace(strings.TrimPrefix(node, "ip"), "_", ".", -1)
+
+			if net.ParseIP(ip) == nil {
+				// Not a valid IP, so treat this is a hostname.
+			} else {
+				node = ip
+			}
+		}
+
+		// Validate that the node actually exists in the network
+		// TODO:
+
+		// Only proxy http for now
+		req.URL.Scheme = "http"
+
+		req.URL.Host = node
+	}
+
+	return &httputil.ReverseProxy{Director: director}
+}