Expose the current spine item using a getCurrentSpineItem() getter method on reflowable, scrollable and one page views.

Hi!

The refactoring introduced in commit d61e9e5 has broken readium-js which accessed the `currentSpineItem` property of the view directly from the `iframe_zip_loader`. 

Since `_currentSpineItem` is a private property now, I've added a getter method so that it remains accessible to readium-js.