message: Ensured replace navigation waits for back image
  Same as for forward navigation
diff: NVSceneController *controller = [[NVSceneController alloc] initWithScene:scene];
  NSMutableArray *controllers = [NSMutableArray arrayWithArray:_navigationController.viewControllers];
  [controllers replaceObjectAtIndex:crumb withObject:controller];
  - [_navigationController setViewControllers:controllers animated:animate];
  + NVNavigationBarView *navigationBar = [self findNavigationBar:scene];
  + void (^completeNavigation)(void) = ^{
  + navigationBar.backImageDidLoadBlock = nil;
  + [self->_navigationController setViewControllers:controllers animated:animate];
  + };
  + if (!navigationBar.backImageOn) {
  + completeNavigation();
  + } else {
  + navigationBar.backImageDidLoadBlock = completeNavigation;
  + dispatch_after(dispatch_time(DISPATCH_TIME_NOW, .1 * NSEC_PER_SEC), dispatch_get_main_queue(), ^{
  + if (navigationBar.backImageDidLoadBlock)
  + completeNavigation();
  + });
  + }
  }
  }
  