StatefulViewController Versions Save

Fixes an issue with Xcode 8.3 (thanks @gunterhager)

Compatibility with Swift 3

• Add Swift 2.3 compatibility

• Adds a new StatefulPlaceholderView protocol placeholder views may adopt in order to set custom edge insets
• Adds a completion handler to setupInitialViewState() (thanks @philippeauriach)

Adds support for tvOS

StatefulViewController is now defined as a generic protocol, with default implementations for UIViewController and UIView. This is a major change that makes it possible to also adopt StatefulViewController for custom UIViewController subclasses.

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