Misc fixes

- eliminate redundancy in Vagrantfile
- Do `vagrant box add` in the Makefile, completing the automation.
- Create a `working` subdirectory that is specifically for "dirty" working images being used (as opposed to the pristine, read-only desktop boxes.

So if you need a raring image in a hurry, now all you need to do is `make raring; make -C working raring` and in a short amount of time that will directly boot right into a graphical Unity session in VirtualBox, using the latest raring packages.