solved bug2----which is to press "更多" when on "BookDetailViewController" and log out, then press "我的书库", and the book status remains unchanged;
modified version----the book status changed into "添加至我的书库" && “此书暂不可借”, when press "添加至我的书库", an alert "您尚未登录，请先登录" shows; meanwhile, if you press "看看谁有这本书", the PreLoginView shows !!!
discard last commit ---- when logging out, just pop detailViewController, discard the if statement in ViewWillAppear, but leave the if statement at "changeExistence" method to make sure one can't launch a addBookRequest when currently logged out
made sure every subViewController on a navigation stack get the notice to popToRootViewController when logging out