Add @objc to Flipbook class for easy integration with Objective-C projects

[obuseme]

It was no problem doing this myself in my own local copy, though would be annoying to keep doing it with no projects.

[frosty]

I've just tested this on a fresh Objective-C project, and I can use Flipbook just fine without having to add an @objc modifier - I think because the Flipbook class itself inherits from NSObject. Could you confirm exactly where this was giving you an issue? Thanks.