title: Back button not appearing in UIViewController
body: <p>I have a <code>UITableView</code> inside my entry-point <code>UIViewController</code> (VC_A). I have another <code>UIViewController</code> (VC_B) that has details for each entry in the <code>UITableView</code>. I have a Show segue from VC_A to VC_B that's triggered in code when a <code>UITableViewCell</code> is pressed:</p>
  
  <pre><code>var passedCellData: CellData!
  func tableView(tableView: UITableView, didSelectRowAtIndexPath indexPath: NSIndexPath) {
      print("it worked!")
      let currentCell = tableView.cellForRowAtIndexPath(indexPath) as! CustomCell
      passedCellData = currentCell.data
      self.performSegueWithIdentifier("OpenDetailSegue", sender: self as UIViewController)
  }
  </code></pre>
  
  <p>(Note that <code>CustomCell</code> extends <code>UITableViewCell</code> and <code>passedCellData</code> is a global variable.)</p>
  
  <p>My custom view controller for VC_B has a <code>data</code> field, so I popular that in <code>prepareForSegue</code>:</p>
  
  <pre><code>override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
      if (segue.identifier == "OpenDetailSegue") {
          let svc = segue.destinationViewController as! DetailController
          svc.data = passedCellData
      }
  }
  </code></pre>
  
  <p><code>DetailController</code> is the <code>UIViewController</code> for VC_B.</p>
  
  <p>I don't have a <code>UINavigationController</code> set up in Storyboard or anything, and both <code>UIViewController</code> instances have <code>UINavigationBar</code> instances at the top (to set the title). However, there's no Back button in VC_B; its navbar only shows the title (and so once I tap for details, I can't go back).</p>
  
  <p>How do I fix this?</p>