Support SkyDNS' stubzones

This implements stubzones in the same way as SkyDNS. This
also works with multiple configured domains and has tests.
Also add more configuration parameters for TLS and path prefix and
enabling stubzones.  Run StubUpdates as a startup command to keep up to
date with the list in etcd.

middleware/etcd/etcd.go

@@ -18,11 +18,12 @@ import (
 type Etcd struct {
 	Next       middleware.Handler
 	Zones      []string
+	PathPrefix string
 	Proxy      proxy.Proxy // Proxy for looking up names during the resolution process
 	Client     etcdc.KeysAPI
 	Ctx        context.Context
 	Inflight   *singleflight.Group
-	PathPrefix string
+	Stubmap    *map[string]proxy.Proxy // List of proxies for stub resolving.
 }
 
 // Records looks up records in etcd. If exact is true, it will lookup just