Add state.SizeAndDo()

This methods returns an OPT record which can be used to create a new
message with the same bufsize and Do bit as the original one.

Working as intended and now tested.

Thus record only diffing in the Key were seen as identical and not
included in the end results. Of course this being a map when and if
this would happen was rather random.

Fixed by including Key. Further small tweaks and code refactors.

Chaos middleware

This allows for CH TXT queries that return some information about
the server and/or the authors (or whatever you put in there).

Closes #41

This adds *most* of the tests from SkyDNS, things lacking is
the stubzone checking, groups and the trim prefix. These
will be added in subsequent PRs and in separate test files.

Etcd stub

SkyDNS can forward requests from one instance to another.
Add this base infrastructure for this feature to CoreDNS.

Add more tests as well.

Rewrite the name of the query as well

the docs a little.

Middleware/loadbalance

This middleware allows you to round robin a/aaaa records in a reply and
maybe more in the future (i.e.) sort a packet?

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