Support PTR records

Add support for PTR records.

PTR records come with basically no strings attached, they're like CNAMEs but don't have any expected behaviour associated – they're _nothing_ but an alias.

PTR records are commonly used for reverse lookups – but that's just a convention that's arisen, it's not the only use for them. For example, some [DNS service discovery approaches](http://www.ietf.org/rfc/rfc6763.txt) use them in other ways.

This does _slightly_ stricter validation than some other record types – checking for empty values.

(I do wonder if we shouldn't do the same zero-labels check for others, like CNAME)
So i've added a test to check this failure happens locally, and it doesn't. PTR records need to be fully qualified, so "bar" is invalid. I'd suggest making use of the `dns.IsFqdn` function here, too.