I'm using `electron.systemPreferences.subscribeNotification` to listen for events coming from `NSDistributedNotificationCenter` in my app:

``` js
prefs.subscribeNotification('com.app.name.EventName', function(notification){
  console.log(notification)
})
```

This produces a single line of output in my log every time the event happens on my system, the only trouble is that the output is `undefined`. My expectation is that I would have access to the `NSNotification` object metadata as JSON.

When I subscribe to this notification in an Objective-C app, I get metadata with the event

``` obj-c
<0x618000000750>-[AppDelegate notify:]: object: com.app.name name: com.app.name.EventName userInfo: {
    Key = Value;
    ...
}
```

atom/browser/api/atom_api_system_preferences.h
#ifndef ATOM_BROWSER_API_ATOM_API_SYSTEM_PREFERENCES_H_
#define ATOM_BROWSER_API_ATOM_API_SYSTEM_PREFERENCES_H_
#include <string>
#include "atom/browser/api/event_emitter.h"
#include "base/callback.h"
#include "native_mate/handle.h"
namespace atom {
namespace api {
class SystemPreferences : public mate::EventEmitter<SystemPreferences> {