1.在BaseNavigationController中 ：
UIImage *backButtonImage = [[UIImage imageNamed:@"返回"] imageWithRenderingMode:UIImageRenderingModeAlwaysOriginal];
self.navigationBar.backIndicatorImage = backButtonImage;
self.navigationBar.backIndicatorTransitionMaskImage = backButtonImage;
2.在BaseViewController中：
self.navigationItem.backBarButtonItem = [[UIBarButtonItem alloc] initWithTitle:@"" style:(UIBarButtonItemStylePlain) target:self action:nil];
