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/proxy/lookup_test.go

@@ -0,0 +1,34 @@
+package proxy
+
+import (
+	"io/ioutil"
+	"log"
+	"os"
+	"testing"
+
+	"github.com/miekg/coredns/middleware"
+	"github.com/miekg/dns"
+)
+
+func TestLookupProxy(t *testing.T) {
+	// TODO(miek): make this fakeDNS backend and ask the question locally
+	log.SetOutput(ioutil.Discard)
+	defer log.SetOutput(os.Stderr)
+
+	p := New([]string{"8.8.8.8:53"})
+	resp, err := p.Lookup(fakeState(), "example.org.", dns.TypeA)
+	if err != nil {
+		t.Error("Expected to receive reply, but didn't")
+	}
+	// expect answer section with A record in it
+	if len(resp.Answer) == 0 {
+		t.Error("Expected to at least one RR in the answer section, got none")
+	}
+	if resp.Answer[0].Header().Rrtype != dns.TypeA {
+		t.Error("Expected RR to A, got: %d", resp.Answer[0].Header().Rrtype)
+	}
+}
+
+func fakeState() middleware.State {
+	return middleware.State{W: &middleware.TestResponseWriter{}, Req: new(dns.Msg)}
+}