create keep_alive cmd for the interpreter

In shell/shell.py inside class Beer2BeerShell add two new functions:

• do_keep_alive()
• help_keep_alive()

Until now we don't have any implementation for keep alive in client code so first function will just do nothing but we can start to write documentation in help_keep_alive()

    def help_keep_alive(self):
        help_string = """
        DESCRIPTION:
            Send a keep alive message to the indexing server just to maintain an active status in the network.

        USAGE:
            >keep_alive
        """
        print(help_string)