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/exchanger.go

@@ -0,0 +1,18 @@
+package proxy
+
+import (
+	"github.com/miekg/coredns/request"
+	"github.com/miekg/dns"
+)
+
+// Exchanger is an interface that specifies a type implementing a DNS resolver that
+// can use whatever transport it likes.
+type Exchanger interface {
+	Exchange(request.Request) (*dns.Msg, error)
+	SetUpstream(Upstream) error // (Re)set the upstream
+	OnStartup() error
+	OnShutdown() error
+	Protocol() protocol
+}
+
+type protocol string