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)
@sivolobov right now the code of this tool is a bit mess because this is just the first import of the previous code, which in turn it was just an experiment.

Now that this is official, and as pointed by @stof, we need to follow the official coding standards of Symfony, which you can get here: http://symfony.com/doc/current/contributing/code/standards.html

Thanks for your help with this PR.