`onclick` property does not work

The `onclick` property of a notification instance doesn't work. 

``` javascript
let Notification = require('node-mac-notifier');
let n = new Notification('test');
n.onclick = function() { console.log('onclick'); };
n.addEventListener('click', () => { console.log('event listener');  });
```

The `addEventListener` callback gets called but not the `onclick`. 
