Also check for double quoted strings in TXT/SPF records

Add specific validator for double quoted CAA records

Support CAA records

Fix empty zone behaviour for SLAVE zones

Add PowerDNS >= 3.5 compatability

Allow new long tlds

Allow underscores in RHS record content

Merge branch 'release/1.2.1'

Release 1.2

Added:
* Unit tests, partially covering the code
* Extended NAPTR validation
* Documentation fixes

Previous Unit Tests base