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/ds_file_test.go

@@ -57,7 +57,7 @@ func TestLookupDS(t *testing.T) {
 
 	log.SetOutput(ioutil.Discard)
 
-	p := proxy.New([]string{udp})
+	p := proxy.NewLookup([]string{udp})
 	state := request.Request{W: &mtest.ResponseWriter{}, Req: new(dns.Msg)}
 
 	for _, tc := range dsTestCases {