First cut at adding vbox_additions to boot2docker.

Right now (with this patch) they are working inside docker containers (like the one used to build)
But not within the main OS.  Using xorriso osirrox to extract the official ISO.

This is not something I consider mergeable right now (although it seems to do no harm) as the kernel modules load, it's just that the service doesn't start.  It could be a 32bit userspace issue which would be sad, but I think it's more likely just a permissions issue?  Pushing this up to see if anyone else can take it on, as I'm not going to be able to work on it for a while.