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/auto/setup.go

@@ -153,7 +153,7 @@ func autoParse(c *caddy.Controller) (Auto, error) {
 					if err != nil {
 						return a, err
 					}
-					a.loader.proxy = proxy.New(ups)
+					a.loader.proxy = proxy.NewLookup(ups)
 
 				default:
 					t, _, e := file.TransferParse(c, false)