+ default credentials support

https://googleapis.dev/python/google-auth/latest/reference/google.auth.html#google.auth.default

https://developers.google.com/identity/protocols/application-default-credentials

sync_ics2gcal/manage_calendars.py

@@ -86,8 +86,7 @@ def main():
     if 'logging' in config:
         logging.config.dictConfig(config['logging'])
 
-    srv_acc_file = config['service_account']
-    service = GoogleCalendarService.from_srv_acc_file(srv_acc_file)
+    service = GoogleCalendarService.from_config(config)
 
     if 'list' == args.command:
         list_calendars(service)