Added ability to fetch all available versions. Also there is method to get latest version.

Now we can provide user with all available versions for symfony.
It includes some exotic version numbers like `2.5.0-BETA2` that is also available for download (http://get.symfony.com/Symfony_Standard_Vendors_2.5.0-BETA2.zip)
Please don't change coding standards in the project, especially not when your change does not respect the Symfony CS