SWIFT | Personalizzare pulsante back in Navigation Controller

override func viewDidLoad() {
        super.viewDidLoad()

        let _btBack = UIButton(type: .Custom)
        _btBack.addTarget(self, action: "returnToRoot:", forControlEvents: UIControlEvents.TouchUpInside)
        _btBack.setImage(UIImage(named: "arrow-left")?.imageWithRenderingMode(.AlwaysTemplate), forState: .Normal)
        _btBack.imageView?.tintColor = UIColor.whiteColor()
        _btBack.sizeToFit()
        
        self.navigationItem.leftBarButtonItem = UIBarButtonItem(customView: _btBack)
        self.navigationController?.setNavigationBarHidden(false, animated:true)
}

func returnToRoot() {
        if let navContr = self.navigationController {
            navContr.popViewControllerAnimated(true)
        }
}