middleware/proxy: implement Exchanger (#480)

By defining and using an proxy.Exchanger interface we make the proxy
more generic and we can then fold back httproxy into proxy.

This overrides #463 and #473 and should make futures extensions rather
trivial

* Add docs that talk about `protocol` and how to set it.
* middleware/proxy: rename New to NewLookup
  It's used as a Lookup mechanism not as a completely new proxy,
  reflect that in the name.
* Set maxfails to 3 by default when looking up names.

Most of the changes have been copied
from https://github.com/johnbelamaric/coredns/pull/1/files

test/cache_test.go

@@ -56,7 +56,7 @@ func TestLookupCache(t *testing.T) {
 
 	log.SetOutput(ioutil.Discard)
 
-	p := proxy.New([]string{udp})
+	p := proxy.NewLookup([]string{udp})
 	state := request.Request{W: &test.ResponseWriter{}, Req: new(dns.Msg)}
 
 	resp, err := p.Lookup(state, "example.org.", dns.TypeA)
@@ -65,7 +65,7 @@ func TestLookupCache(t *testing.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")
+		t.Fatal("Expected to at least one RR in the answer section, got none")
 	}
 
 	ttl := resp.Answer[0].Header().Ttl