Added ability to set custom data in the APNS adapter

Allows setting custom data (I think this was already implemented before but it seems like it's disappeared in v2).

````php
$message = new Message('This is a basic example of push.');
$message->setOption('custom', $content);
````

I haven't added a special method for this, rather just using the `setOption` method. Is this OK or would you prefer a new method to be defined on a `Message`?