Added command to remove AcmeDemoBundle

Bug fix: no
Feature addition: yes
Backwards compatibility break: no
Fixes the following tickets: -
Todo: -
License of the code: MIT
Documentation PR: symfony/symfony-docs#1692

This PR adds a demo command to the Acme Bundle as well as allows new users to get rid of the AcmeDemoBundle once they want to start coding. I've found myself many times needing such a feature and I guess I'm not the only one out there that could use it.