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/file/setup.go

@@ -110,7 +110,7 @@ func fileParse(c *caddy.Controller) (Zones, error) {
 					if err != nil {
 						return Zones{}, err
 					}
-					prxy = proxy.New(ups)
+					prxy = proxy.NewLookup(ups)
 				}
 
 				for _, origin := range origins {