Add AUTODNSSEC, implement for DNSimple (#640)

* Add AUTODNSSEC, implement for DNSimple

There are two models for DNSSEC in DNS management: either dnscontrol
manages all the DNSSEC records and has to be invoked regularly for
re-signing, or the provider manages DNSSEC automatically and dnscontrol
is not involved beyond saying "yeah, do that".

This implements the latter, as a model, and for one provider.

Potentially we could tune/configure options for DNSSEC such as
algorithm, but DNSimple don't expose that API so I haven't implemented
it.

This minimal model should be something which maps into other providers
cleanly.

* Fix missing CanAutoDNSSEC on provider

* Validation fix for master broken

This is broken in master and causing Travis in my branch to fail.  The
validation tool runs with `gofmt -s` to require "simplify", and so
rejects an ignored second bound variable to range iteration.

* Correct wire in the AUTODNSSEC validation step

models/record.go

@@ -28,6 +28,7 @@ import (
 //     TXT
 //   Pseudo-Types:
 //     ALIAS
+//     AUTODNSSEC
 //     CF_REDIRECT
 //     CF_TEMP_REDIRECT
 //     FRAME