Add canonical sorting

Add a low-allocation canonical sorting function that can be used in
middleware.

This implements the tests from rfc4034 correctly.

Transfer out

These can be enabled by adding "transfer out" to the Corefile. Without
it no AXFR is allowed.

For now only AXFR and no IXFR. No TSIG and no ACLs.

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.