Add testing package

This already includes a bunch of helper functions that aid in testing.
Factor out etcd and file testing to use this package.

Fixes: #50

Etcd stub

Zonefile fix

Full implementation, DNS (and in the future DNSSEC). Returns answer in a
hopefully standards compliant way.
Testing with my miek.nl zone are included as well.
This should correctly handle nodata, nxdomain and cnames.

This implements stubzones in the same way as SkyDNS. This
also works with multiple configured domains and has tests.
Also add more configuration parameters for TLS and path prefix and
enabling stubzones.  Run StubUpdates as a startup command to keep up to
date with the list in etcd.

Cname ordering

Add a test for SkyDNS':
https://github.com/skynetservices/skydns/issues/217

Put the CNAME in front for both answer and extra sections. Note that
the etcd middleware seems to already to the correct thing though.

Add Scrub function

This function will make the message fit for the client's buffer, or
set the TC bit.

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