use Notification Center to pop LoginController when Login finished
added alert when network is broken--Attention: must determine the error kind before raise an alert, cuz when a url connection is interrupted, an alert is also raised!
minor changes, doesn't effect the function
added SearchTableViewController in storyboard and created SearchTableViewController.h/m, next is to implement .m file
added foundations of SearchTableViewController, next is to implement the real search process
decided not to use framework now, created JsonDataFetcher to fetch searched json results
added DataConverter to convert fetched jsonData into Book objects