Printing node->nsec3_node + sample NSEC3 zone file

- Added sample zone file for example.com zone signed with NSEC3.

refs #425

samples/example.com.zone

 $TTL 1h                  ; The default expiration time of a resource record without its own TTL value
 $ORIGIN example.com.
 @  IN  SOA  ns.example.com. username.example.com. (
-              2007120711 ; serial number of this zone file
+              2007120712 ; serial number of this zone file
               1d         ; slave refresh (1 day)
               1d         ; slave retry time in case of a problem (1 day)
               4w         ; slave expiration time (4 weeks)

src/dnslib/debug.c

@@ -145,6 +145,16 @@ void dnslib_node_dump(dnslib_node_t *node, void *data)
 		printf("none\n");
 	}
 
+	printf("NSEC3 node: ");
+
+	if (node->nsec3_node != NULL) {
+		name = dnslib_dname_to_str(node->nsec3_node->owner);
+		printf("%s\n", name);
+		free(name);
+	} else {
+		printf("none\n");
+	}
+
 	printf("RRSet count: %d\n", node->rrset_count);
 
 	dnslib_rrset_t *tmp = (dnslib_rrset_t *)skip_node->value;