Provide a ttl for associated txt records

Pass in a ttl argument to the Record call when setting the TXT records.
Otherwise, the TXT records always end up with the default 3600s TTL value
from https://github.com/appliedsec/Powerglove-DNS/blob/master/powerglove_dns/model.py#L33
Merge pull request #4 from jude/patch-3

Provide a ttl for associated txt records