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

middleware/proxy/upstream_test.go

@@ -165,6 +165,20 @@ proxy . 8.8.8.8:53 {
 proxy . some_bogus_filename`,
 			true,
 		},
+		{
+			`
+proxy . 8.8.8.8:53 {
+	protocol dns
+}`,
+			false,
+		},
+		{
+			`
+proxy . 8.8.8.8:53 {
+	protocol foobar
+}`,
+			true,
+		},
 	}
 	for i, test := range tests {
 		c := caddy.NewTestController("dns", test.inputUpstreams)