Adding vagrant config files

This adds Vagrant config files to Symfony2 Standard Edition.

It uses our (= Liip) Vagrant Base Box, which is basically just a Debian-6.0.3-amd64-netboot image.
It could be done with other base boxes as well, but it was the easiest way for me.

What do you think about adding this? I'm also open to another approach, but I think having "the vagrant way" as an addition to check out Symfony2 Standard Edition would be great for beginners (or people who just want to have a quick look).