改 navbar title color 一劳永逸 的方法

NSMutableDictionary *titleBarAttributes = [NSMutableDictionary dictionaryWithDictionary: [[UINavigationBar appearance] titleTextAttributes]];
[titleBarAttributes setValue:navBarTitleColor forKey:UITextAttributeTextColor];
[titleBarAttributes setValue:[NSValue valueWithUIOffset:UIOffsetMake(0.f, 0.f)] forKey:UITextAttributeTextShadowOffset];
[[UINavigationBar appearance] setTitleTextAttributes:titleBarAttributes];