title: how to replace/customize back button image in storyboard navigationcontroller
body: <p>I want to replace the text in the back button with a custom image.  How can I do that in swift code?  I don't want to replace the entire backbarbutton since I'd like to keep the default action behaviour of going back to the last view.  I would also like to do a switch statement based on where the back destination(based on storyboardId) is so that I can show different images based on the view.</p>
  
  <p>this replaces the image but it wipes out the default back button behavior, and I need to discrimante on what the back destination is so that I can show the right back image.</p>
  
  <pre><code>self.navigationItem.leftBarButtonItem = 
   UIBarButtonItem(image:StyleKit.imageOfMap, style:.Plain, target:self, action:nil);
  </code></pre>