Perform a generic POST-request to a target server, sending the sub-store postParameters from the value store.
- All values that are to be send should be set or moved into
postParameters first.
- postURL: url of the server to send the request to
- On success: The request has been performed successfully, the server answered with 'ok'.
- On fail: The request has been performed successfully, the server answered with 'error'.
- On missing url: No target url has been provided.