Improve "About" command contents, which are used as the installer help

Improved the contents of the "About" command which is used as the installer help

**Before**

![before](https://cloud.githubusercontent.com/assets/73419/5103605/d7989eb2-6fd9-11e4-95fa-5efd7b8a89d7.png)

**After**

![after](https://cloud.githubusercontent.com/assets/73419/5103608/da4d574c-6fd9-11e4-95a2-fa3b878f313a.png)
rather than hardcoding ``symfony`` in it, it would be better to detect the name used by the user when installing it. This way, the message would be right even for users using a different setup (for instance a local phar rather than a global ``symfony`` executable).

The Command class uses ``$_SERVER['PHP_SELF']`` to get it when replacing ``%command.full_name%`` in the command help.