Install napa as a dependency

I actually hadn't heard about napa before, but from a quick look I think it's brilliant and will be using in the future :)

Just one potential improvement if you are interested, might be to install napa from npm as part of your module (it might sit better in devDependencies in the long term) and then use the local version to run the install.  In cases where people (like myself) don't have napa installed globally an `npm install` operation will still work.