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/setup.go

@@ -20,7 +20,7 @@ func setup(c *caddy.Controller) error {
 		return middleware.Error("proxy", err)
 	}
 	dnsserver.GetConfig(c).AddMiddleware(func(next middleware.Handler) middleware.Handler {
-		return Proxy{Next: next, Client: newClient(), Upstreams: upstreams}
+		return Proxy{Next: next, Upstreams: upstreams}
 	})
 
 	c.OnStartup(OnStartup)