VBox guest additions

This is a PR in response to #282.
It adds ~400kb to the unpacked system.
Please discuss wether vboxsf are a good option of not in #282, not here.

Add a shared folder to the VM:
```
$ boot2docker stop
$ VBoxManage sharedfolder add boot2docker-vm -name home -hostpath $HOME
```

Now mount the folder inside the VM:
```
$ boot2docker up
$ boot2docker ssh "sudo modprobe vboxsf && mkdir -p $HOME && sudo mount -t vboxsf home $HOME"
```

You can now run `docker -v` transparently as long as the volume is inside `$HOME`.

A test ISO based on 0.7 is at https://dl.dropboxusercontent.com/u/12014139/boot2docker.iso