Added NotificationCenter post when keys have finished synchronization

[twostraws]

This adds support for applications being notified when Zephyr has finished synchronizing iCloud changes. It's done with two changes:

1. It adds a static keysDidChangeOnCloudNotification notification name that anyone can observe.

2. It posts that notification in the keysDidChangeOnCloud() method when synchronization has finished.

So, if an app wants to be informed when synchronization has completed, they would write code like this:

NotificationCenter.default.addObserver(self, selector: #selector(cloudDataChanged), name: Zephyr.keysDidChangeOnCloudNotification, object: nil)

Unrelated but part of this PR: I've added an import for UIKit at the top – Xcode complains if I don't have this, because there's already an observer for UIApplication.willEnterForegroundNotification and UIApplication comes from UIKit.

[ArtSabintsev]

This looks good to me - thank you! I'll merge it and cut a new version when I'm back home this evening.

[ArtSabintsev]

Released as v3.4.0 just now.