Changes to Protocol and AppID

Accepted PRs

#54, #55, #56

TL;DR

• Siren uses your app's bundle identifier to perform a version check. The appID param is now used by Siren under the hood. It'll throw an error if you try to set it as it's privately scoped.
• SirenDelegate now sets default empty implementations of all six methods. Therefore, the @objc attribute and optional keyword have been removed as all SirenDelegate methods are now optional by default.
• Signature change from sirenDidShowUpdateDialog() to sirenDidShowUpdateDialog(alertType: SirenAlertType)

Long Version

• As of v0.8.0, Siren is now able to check if your users are using the current version of your app using the app's bundle identifier, which is accessible through the NSBundle API. Instead of removing/deprecating appID, it is still storing your app's iTunes Connect appID, but it is now determined the appID dynamically from the results of the iTunes Lookup API call. Once it's determined, the appID is stored within Siren and used to launch the App Store page of your app when needed.
• Removed @objc attribute from SirenDelegate.
• Removed optional keyword from all SirenDelegate methods.
• SirenDelegate now sets default empty implementations of all six methods.
• Signature change from sirenDidShowUpdateDialog() to sirenDidShowUpdateDialog(alertType: SirenAlertType)