Make popover resize according to the preferredSizes of ViewControllers

Auto-adjust popover size based on preferredSize of topViewController

// Conform to UINavigationControllerDelegate and then implement this:
// Especially if the topViewController is a RBStoryboardLink or something.

- (void)navigationController:(UINavigationController *)navigationController willShowViewController:(UIViewController *)viewController animated:(BOOL)animated {
    // NB: Using navigationController.preferredContentSize would be better here, but it doesn't reset properly when navigating back so we manually add on the height of the navbar instead.

    CGSize size = viewController.preferredContentSize;
    CGFloat navbarHeight = navigationController.navigationBar.frame.size.height;

    <topViewController>.preferredContentSize = CGSizeMake(size.width, size.height + navbarHeight);
}