Better API for adding buttons to form

Allows following syntax:

```php
$form->addPaymentButtons(array(
   $this->processForm,
));
```

Or even:

```php
use Markette\Gopay;

$form->addPaymentButton(Gopay\Service::METHOD_GOPAY)
    ->onClick[] = $this->processGopayMethod;
```
yes, remove it ! :) we will try :D