Fixed bug with added namespace to GoPay API

The GopayHelper class is added to a namespace in this project. Therefore, it must declare to use \Exception from global namespace, which I have fixed. Before this patch, the code throws an error about unknown class \Markette\Gopay\Api\Exception.
Fixed bug with dumped variable in return payment

The ReturnPayment::isPaid method dumped the variable in its body. That caused the variable to be displayed on top of the page, unless the redirect is performed, after the return payment processing. It works correctly now and displays nothing.
Fixed the namespace errors with the newest Nette version

Nette changed a namespace for the class used in GoPay Extension. I have fixed the namespace. In order not to encounter such problem again, composer dependency to Nette has been changed to ~2.1. If Nette development sticks to semantic versioning, any (even minor) API changes should be postopned until the version ~2.2.