Added a new self-update command to easily update the installer

use ``%command.full_name%`` instead of ``php symfony %command.name%``. This way, it will avoid hardcoding the way the phar is access (when using a global install, ``php symfony`` would not work btw, and a local install would probably be ``php symfony.phar`` instead, so your help text does not work for common cases).

Btw, I'm not sure it is needed to display it. The HelpCommand already shows the way to call the command at the beginning of the output when showing the signature. Given there is no options, it will be exactly the same