Swift version of viewWillDisappear needed for custom storyboard back button behavior

viewWillDisappear

override func viewWillDisappear(animated:Bool) {
    if let currentViewControllers = self.navigationController.viewControllers as? [UIViewController] {
        if let found = find(currentViewControllers, self) {
            // Do nothing if the current controller is still in the stack. This means we're going forward, not back
        }
        else {
            self.performSegueWithIdentifier("unwindToLogin", sender:self)
        }
    }
    super.viewWillDisappear(animated)
}