add Safari Push Notifications support.

Here are the changes to support Safari Push Notifications:
* add `AppleSafariNotificationPayload` class which inherits `AppleNotificationPayload`.
* change `AppleNotificationPayload.ToJson()` method as virtual.
* add new rules to `ApplePushChannelSettings.DetectProduction()` and `ApplePushChannelSettings.CheckProductionCertificateMatching()`.
* add Safari Push Notification sample code to `PushSharp.Sample` project.