AppleNotification - Increased Max Payload Size.

Apple recently increased Max Payload Size from 256 to 2048 bytes (2K) on both sandbox and production APN servers. Since it's a server upgrade, this change will also affects all previous iOS versions. I changed the MAX_PAYLOAD_SIZE accordingly. I recommend doing so because the library internally check payload size, raising a NotificationFailureException if it's lesser than MAX_PAYLOAD_SIZE const and thus preventing any notification greater than 256 bytes to be sent.