Extract ensure stage into it's own npub verify command

For projects using coffeescript it's nice to be able to do the following in your `package.json`:

    "scripts": {
      "pretest": "make build",
      "test": "echo 'some testing stuff!' && exit 0",
      "posttest": "npub verify"
    },

This will catch any failures to build `src` or copy license headers from `src` to `lib` during the publish process.

_This will cause a minor merge conflict with #7 that I'm happy to sort out. It shouldn't matter which order these get merged._