Added AppcuesFirebaseMessagingService - This allows customers to plugin their firebase "app" into their app, work as an abstraction for how messaging works when sent from Appcues only.
Added Static method calls to AppcuesFirebaseMessagingService - Allows customers to have multiple messaging services (calling static methods that will return true if its a handled message or false when its ignored)
Added Initial Debugger implementation for Push messages - Pending to add more error messages, but the initial structure checks if push message is sent
Added XML config options - Initial options that customers can override with their own settings.
Modified sample app to implement AppcuesFirebaseMessagingService.