Added support for virtual box executor

Added support for virtual box as an executor.

I didn't see any progress being made on gitlab-org/gitlab-ci-multi-runner#159.

I tried writing this first with the libvirt libraries. Unfortunately, there were numerous problems with what libvirt supports and what I had available for testing. I switched to using vboxmanage on the backend and stopped running into the problems. I hope that's okay.

This is my first dabble with Go, so please advise if there's better methods for what I did. I tried to stick close to the same layout that was used in the Parallels executor. I'm following Vagrant's lead on SSH configuration, so it uses port forwarding starting at port 2222.

No additional libraries are needed as long as virtualbox is installed.