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.md

@@ -13,7 +13,7 @@ other servers in the network.
 etcd [zones...]
 ~~~
 
-* `zones` zones it should be authoritative for.
+* `zones` zones etcd should be authoritative for.
 
 The will default to `/skydns` as the path and the local etcd proxy (http://127.0.0.1:2379).
 If no zones are specified the block's zone will be used as the zone.
@@ -21,15 +21,21 @@ If no zones are specified the block's zone will be used as the zone.
 If you want to `round robin` A and AAAA responses look at the `loadbalance` middleware.
 
 ~~~
-etcd {
+etcd [zones...] {
+    stubzones
     path /skydns
     endpoint endpoint...
-    stubzones
+    upstream address...
+    tls cert key cacert
 }
 ~~~
 
-* `path` /skydns
-* `endpoint` endpoints...
-* `stubzones`
+* `stubzones` enable the stub zones feature.
+* `path` the path inside etcd, defaults to "/skydns".
+* `endpoint` the etcd endpoints, default to "http://localhost:2397".
+* `upstream` upstream resolvers to be used resolve external names found in etcd.
+* `tls` followed the cert,  key and the CA's cert
+
+TODO: TLS params!
 
 ## Examples