Implement `do_register()` logic inside `shell/shell.py/`

Once https://github.com/kinderp/beer2beer/issues/13 has been completed we need to implement in shell/shell.py logic inside do_register().

You can see do_login() as reference, here but summarizing you need to:

1. Build a correct register payload (see here for deails) using function in https://github.com/kinderp/beer2beer/issues/13
2. Create a CommandRegister
3. Call execute() method

register_payload =  CALL UTIL FUNCTION for a list of shared files!!!
register_command = CommandRegister(
                ShellSettings.SERVER_HOST, ShellSettings.SERVER_PORT, register_payload
)
response = register_command.execute()