Support Safari push notification

[jonas0616]

Safari push notification is basically based on APNS. The only difference is the format of JSON dictionary of notification body:

{
    "aps": {
        "alert": {
            "title": "Flight A998 Now Boarding",
            "body": "Boarding has begun for Flight A998.",
            "action": "View"
        },
        "url-args": ["boarding", "A998"]
    }
}

However, the existing ApnsPHP does not allow non-string alert content. And it needs a method to add url-args.

[delitestudio]

@jonas0616 We have implemented a custom message that inherits from Message and handles the Safari payload. We have sent a pull request with this integration.