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)
I know and so I added the comment. @javiereguiluz has the line
> The Symfony version should be 2.N.M, where N = 0..9 and M = 0..99
And so I thought that it is known for someone that versions will not be over 2.9