Add etcd middleware

This middleware acts in the same way as SkyDNS. We might add options
to allow it to be behave different, but for now it will suffice.

A Corefile like:
.:1053 {
    etcd miek.nl
    proxy . 8.8.8.8:53
}
will perform lookup in etcd and proxy everything not miek.nl to Google
for further resolution.

The internal etcd forwarding *also* uses the proxy infrastructure,
meaning you get health check and such for (almost) free

core/directives.go

@@ -53,10 +53,13 @@ var directiveOrder = []directive{
 	// Directives that inject handlers (middleware)
 	{"prometheus", setup.Prometheus},
 	{"rewrite", setup.Rewrite},
-	{"file", setup.File},
-	{"reflect", setup.Reflect},
 	{"log", setup.Log},
 	{"errors", setup.Errors},
+
+	{"etcd", setup.Etcd},
+	{"file", setup.File},
+	{"reflect", setup.Reflect},
+
 	{"proxy", setup.Proxy},
 }