Logging to syslog of DNS changes made.

Hi

Adding, deleting and editing of records are logged if syslog is enabled.  I've formatted the logging in a way that should be easily machine-parsable (I'm going to be collecting them with logstash).

Any comments welcome.  I hope I've taken a reasonable approach.
If it looks good, I'd perhaps like to add zone deletion and creation.

Thanks for your consideration,
Alex