[iOS7] removing extra padding on bar button items customized using stylesheets.

We tested different methods but we now insert an extra item with negative width when customizing left or right bar button items on iOS 7 as it has been proposed here :

http://stackoverflow.com/questions/18914812/how-to-edit-empty-spaces-of-left-right-uibarbuttonitem-in-uinavigationbar-in-io

Using the extra rect inset method on the custom button wasn't working well when navigating (pushing/popping view controllers in navigation controller).