title: Admob banner in the middle of toolbar
body: <p>Is it possible to put admit banner in the middle of <code>navigationController.toolbar</code> ? Way I did it (inside <code>viewDidLoad</code>):</p>
  
  <pre><code>    let banner = GADBannerView(adSize: kGADAdSizeBanner)
      banner.adUnitID="ca-app-pub-123"
      banner.rootViewController=self
      banner.load(GADRequest())
      self.navigationController?.isToolbarHidden=false
      self.navigationController?.toolbar.addSubview(banner)
      self.navigationController?.toolbar.sizeToFit()
  </code></pre>
  
  <p>However banner is aligned to the left side of a toolbar. Is there a simple way to align banner to the middle?</p>