add 'removeListener' method to have a way to remove the NFC listener

[clickclickonsal]

Fix for Issue #8

[henrikra]

This is not ready yet since you should be able to specify which listeners you want to remove. For example:

class Example extends Component {
  componentDidMount() {
    NFC.addListener(this.logPayload);
  }

  componentWillUnmount() {
    NFC.removeListener(this.logPayload);
  }

  logPayload(payload) {
    console.log(payload);
  }
}

This way I can specify which listener I can remove :)

[henrikra]

What _registeredToEvents variable means in general?

[clickclickonsal]

_registeredToEvents keeps track if the DeviceEventEmitter.addListener is still attached.
This only needs to be set to false if DeviceEventEmitter is removed.
I created a removeAllListeners method because this truly removes the eventListener.
Perhaps I should call removeAllListeners destroy instead?

[clickclickonsal]

The more & more I look at this code, I think it should look something like this instead.

NFC.initialize()
NFC.addListener(name, callback)
NFC.removeListener(name)
NFC.removeAllListeners()
NFC.destroy()