This middleware acts in the same way as SkyDNS. We might add options
to allow it to be behave different, but for now it will suffice.

A Corefile like:
.:1053 {
    etcd miek.nl
    proxy . 8.8.8.8:53
}
will perform lookup in etcd and proxy everything not miek.nl to Google
for further resolution.

The internal etcd forwarding *also* uses the proxy infrastructure,
meaning you get health check and such for (almost) free

Fix early stage warning

I'm guessing this was the intended meaning. =)

Remove things not supported, fix docs.

When we cannot bind to the port, return an error which will make
CoreDNS fail startup. Still a bit hackish, but good enough.

Fix upstream tests in proxy middleware

Errors directive testing and fixing

Drop a few tests and make it work and compile.
Also add the documentation: errors.md

Fix middleware log

Context

Rename the old Context to State and use context.Context in the
middleware for intra-middleware communication and more.