Added boxes for 13.04 32 and 64 bits

Hi. First, thanks a lot for the base boxes, they are really useful :)

I adapted your recipes to add two new versions of Ubuntu, and I put packaged versions of them on copy.com so people will be able to download them. I also made a small adjustment to the instructions on the readme, the newer versions of vagrant with multiple providers need a different command for validate. If you merge I'll send the pull request to vagrantbox.es with the links for the new boxes.

Thanks