Added a new "symfony demo" command

[javiereguiluz]

This is a WIP and early release of the new symfony demo command. The general idea around this command is the following:

• Soon the AcmeDemoBundle will be gone forever.
• The Symfony Standard Edition (and therefore, the symfony new command) will always give you a clean and empty Symfony application.
• We'll publish a complete Symfony demo application that combines what we did for the Best Practices book (and which wasn't released) and the current AcmeDemoBundle. This will be a real application using AppBundle and all the best practices. No more Acme or Demo.
• If you are new to Symfony and want to test it ... install the Symfony Installer in 1 minute and execute the symfony demo command. In 10 seconds you'll have a full-featured Symfony application ready to test.

And here are some comments regarding this command:

• For now it doesn't download anything because the Symfony_Demo package doesn't exist yet.
• This command must be as simple as possible (remember that it will be used by newcomers that just want to test-drive Symfony quickly and easily). That's why we don't allow to select the Symfony version (it will always be the most recent one). And we also don't allow to select the directory where the demo project is installed (it will be installed in getcwd().'symfony_demo/')

And here it is the demo command in action:

[Pierstoval]

👍

I'm gonna use this command in my trainings once it becomes available 👍

[xabbuh]

Just a quick suggestion: Instead of copying a lot of code from the new command, we should probably some parts to a dedicated layer and reuse it in both commands. As a benefit, this would probably also ease testing them quite a lot (I still need to wrap up my pull request though).

[javiereguiluz]

@xabbuh what would you think if I make the DemoCommand inherit from the NewCommand?

[xabbuh]

In this case, an abstract base command class would probably be better.

[javiereguiluz]

@xabbuh I've updated this PR to introduce the common abstract method.

[stof]

$this->fs should be moved to the DownloadCommand as it is used here