[WIP] Docker packaging for Vagrant

This is a first run to package docker in a nice pre-built vagrant box. It's not finished yet but I thought it might be better to avoid duplicating the effort. Feel free to take over as I'm going to be quite busy for a while.

The current build uses [packer](http://packer.io), the build tool developed by the same guy that's behind vagrant. It supports only 3 vagrant targets (VirtualBox, VMWare Fusion, AWS) for the moment but more are surely to come. For now only VirtualBox is tested and VMWare is supposed to work but I didn't have it available on my computer.

The build installs Ubuntu 12.04.3 with the raring (3.8) kernel, a minimal set of dependencies and the new docker 0.6.
Install packer and run `packer --only=virtualbox template.json` to get the output (~280MB)

There's still a couple of things I wanted to check out:
- the .sock file is not accessible from the vagrant user
- play more with the install to make sure docker is working properly
- see if it's an issue that the manpages are missing (this keeps the box lighter)

That's it for now. Feel free to close if you feel this is not needed I won't get offended :)