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/etcd/proxy_lookup_test.go

@@ -16,7 +16,7 @@ import (
 
 func TestProxyLookupFailDebug(t *testing.T) {
 	etc := newEtcdMiddleware()
-	etc.Proxy = proxy.New([]string{"127.0.0.1:154"})
+	etc.Proxy = proxy.NewLookup([]string{"127.0.0.1:154"})
 	etc.Debugging = true
 
 	for _, serv := range servicesProxy {