Rebuilding "A first project" tutorial

[anthumchris]

I struggled with following the tutorial "A first project", because there is no npm task to build the project after modifying source files. Could be helpful if build/watch tasks were added to package.json with usage instructions:

"scripts": {
  "test": "node --napi-modules ./test/test_binding.js",
  "build": "...",
  "build-watch": "..."
},