Fixed that parentViewController was not set on child controllers

If IIViewDeckController is the window's rootViewController then parentViewController is nil and the _finishTransitionBlocks are never called.
This causes the IIViewDeckController's child controllers to have no parentViewController which breaks a lot of stuff like modal screens and unwind segues.

The way parent controllers are connected in ViewDeck is wrong anyway but I didn't fix that. The left, top, right and bottom controllers should always have the IIViewDeckController as parentViewController.