title: Replace left navigation button with custom button, then back
body: <p>I have a <code>UITableView</code> within an <code>UINavigationController</code>. On the right of the <code>UINavigationBar</code> I have an "Edit" button. When I tap this button, all the textfields in the table cells become activated so I can edit them. The right button changes to "Save".</p>
  
  <p>I'd now like the left "back" button of the <code>UINavigationController</code> to be replaced by a "Cancel" button which when I tap doesn't bring me back to the previous <code>UIViewController</code>, but just cancels editing mode.</p>
  
  <p>When I tap "Save" the "Cancel" button should be changed back to the usual <code>UINavigationController</code> back button. Is there any easy way to do this? I tried accessing <code>[[self navigationItem] leftBarButtonItem]</code>. This works for the right button, but not the left one.</p>