Added Stop method

Hi,

I've been using this code to create a NFC Node (https://github.com/hardillb/node-red-contrib-nfc) for the Node-RED project (https://github.com/node-red/) and I needed a way to shut down the NFC reader at redeploy and shutdown so I've added a stop function.

If you could merge this and update the npmjs.org to the latest version it would mean I can stop pointing explicitly at my fork.

Thanks,