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

middleware/etcd/etcd.md

@@ -0,0 +1,33 @@
+# etcd
+
+`etcd` enabled reading zone data from an etcd instance. The data in etcd has to be encoded as
+a [message](https://github.com/skynetservices/skydns/blob/2fcff74cdc9f9a7dd64189a447ef27ac354b725f/msg/service.go#L26)
+like [SkyDNS](https//github.com/skynetservices/skydns).
+
+## Syntax
+
+~~~
+etcd [zones...]
+~~~
+
+* `zones` zones it 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.
+
+If you want to `round robin` A and AAAA responses look at the `round_robin` middleware. optimize
+middleware?
+
+~~~
+etcd {
+    path /skydns
+    endpoint endpoint...
+    stubzones
+}
+~~~
+
+* `path` /skydns
+* `endpoint` endpoints...
+* `stubzones`
+
+## Examples