Add ability to pass a few callbacks to listenTo

Hey, it's sometimes useful to pass a few callbacks to `listenTo` function.

For example:

``` js
// Menu view
Backbone.View.extend({
  initialize: function() {
    // * this.notifications is collection of user notifications;
    // * count of unread notifications should be displayed in 
    //   menu (like "Messages (3)");
    // * if user receive "important" message, for example incoming call,
    //   user should be redirected to page with that call.
    this.listenTo(this.notifications, '', this.updateUnreadCount, this.redirectIfImportant);
  }
});
```

Without ability to pass a group of callbacks we should call `listenTo` twice or write a function, that does two things (what is worse).

Thoughts?