Implement account update method

Allow syncing of all known accounts from PowerDNS, in the same
way that Domain().update() does for domains.

update_zones.py

@@ -29,6 +29,6 @@ with app.app_context():
         sys.exit(1)
 
     ### Start the update process
-    app.logger.info('Update zones from nameserver API')
+    app.logger.info('Update domains from nameserver API')
 
-    d = Domain().update()
+    Domain().update()