Added RFC2308 compliant \$TTL directive behavior. The directive is used to set the TTL of all resource records that do not explicitly set a TTL. When a \$TTL directive is not set, the original behavior of inheriting the previously set TTL is used. When neither a \$TTL directive or explicit RR TTL is set, the minimum TTL of the SOA record is used. An additional test dns-zoneparse-dollar-ttl.t has be added to prove that the original behavior is still working. The main test dns-zoneparse.t has been updated to reflect the new behavior as the test zone test-zone.db uses a \$TTL directive. Both of these tests prove the functionality to work in either case.