Optional permission callback

The callback is redundant in the following use-case:

```javascript
var userWantsChatNoficiations = false;

// This would be called when the user says
// they want notifications for chat messages
function userRequestsChatNotifications() {
  // The callback is redundant in this case
  Notification.requestPermission(function() {});
  userWantsChatNoficiations = true;
}

// User gets a new message
function onNewChatMessage(user, msg) {
  // Check if the user wants notifications & we have
  // permission to show them.
  if (userWantsChatNoficiations && Notification.permission == 'granted') {
    new Notification("Message from " + user, {
      body: msg,
      tag: 'msg_' + user
    }).show();
  }
}
```