main_server.py is a customized HTTP server.
client.py is a client program.
The userList file should be put into the same folder as main_server.py. It stores the user-account information.

To run main_server.py:
    $ python main_server.py [--port=PORT_NUMBER]
To run client.py:
    $ python client.py [--host=HOST_NAME][--port=PORT_NUMBER]

Client users can just follow the commandline information to use the file share system.
    First, you need to register an account.
    Second, use that account to login.
    Then, you can download or upload files.
    *notice: please make sure that the file paths are absolute paths
        For download: all files are stored in '/'. (Example: '/myPicture.png')
        For upload: just use the local absolute paths