This adds the command structure for rbpkg, the new package installer for
Review Board. Right now, there aren't any commands available, but those
will come later.

rbpkg's command structure is modeled heavily off of RBTools.

Some may wonder why we're building our own installer, so I provided a
bunch of rationalization in the README.md.

Ran rbpkg. Got help output.
There are no commands yet, so.. we'll see how well it works in practice!