iOS navigationBar透明效果，不影响barButtonItem

   类似于手机QQ空间的效果；

    直接上代码：

    // 去掉背景图片

    [self.navigationController.navigationBarsetBackgroundImage:[UIImagenew] forBarMetrics:UIBarMetricsDefault];

    // 去掉底部线条

    [self.navigationController.navigationBarsetShadowImage:[UIImagenew]];

    // 添加自定义背景view

    CGRect backView_frame = CGRectMake(0, -20,self.view.bounds.size.width,64);

    UIView *backView = [[UIViewalloc] initWithFrame:backView_frame];

    backView.backgroundColor = [UIColorredColor];

    backView.alpha = 0.0;

    [self.navigationController.navigationBaraddSubview:backView];

    self.bgView = backView;