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
