undid changes from previous commit

fixed an issue that could cause unbalanced transitions in some situations

improved use of context in KVO, allowing to remove KVO when changing value of borrowNavItemContentsFromChildren

Added borrowNavItemContentsFromChildren property, removeViewController: can now accept nil as a parameter without crashing, now calling willMoveToParentViewContoller:nil when transitioning away from a vc

Fixed issue that caused container to switch back to self.viewControllers[0] in viewWIllAppear: if you had already switched to a different view controller in viewDidLoad

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stop observing currentViewController's navigationItem on dealloc

use KVO to be alerted when changes occur to the current child view controller's navigationItem contents. Added animateNavItemBarButtonItemChanges property as a workaround for the fact that the animated parameter is ignored when changing bar button items with a method such as setLeftBarButtonItems:animated:

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improved comment clarity

fixed incorrect code example in comment

grammatical fix in comment

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reformatted code example in readme to better fit page