Added a new package of com.depot.cs4900.network, which contains
    two files. HTTPRequestHelper contains networking related methods that help sending
    four types HTTP requests to the server. And they are GET, PUT, POST, and DELETE.
    Note: In order for this step (Online_Step_1) to work, the depot server needs to be
    updated as well, especially the controllers.
Made several changes. Still working on it...
All the following are working now: login, logout, display of books belonging to the current user. Read the "Comments" file for details.