Add a convenience cleanup method to reduce boilerplate

[pantheraleo-7]

Since the SimpleMFRC522 module is all about convenience and less code, providing this method would

• eliminate the need to import the [relatively low-level] RPi.GPIO library for using just one method
• provide a more object oriented API

of course if the user needs GPIO, they can always import it. Just that it wouldn't be forced by us now.