ZoneRecordData should include CAA records

[WesleyAC]

CAA records are natively supported by BIND 9.9.6 and higher, so I think it'd make sense for them to be supported in the ZoneRecordData type.

[partim]

I had a look at RFC 6844 – it looks like the record and its representation format are slightly more complex, so I would like to postpone adding it until after the 0.10.0 release to not delay it any further. Hope that’s okay.