Improve Developer Ergonomics

[foxtree]

• Use yarn (and yarn.lock) for package management

• Lock-down the node version via .nvmrc (Node v4 is approaching the end of LTS)

• Live-reload the app on code changes

• Enable support for linting, with a watcher

• 4 new tasks/scripts are added to package.json :
  ** start:watch: for development only -- live-reload app on code changes
  ** lint:js: for CI, or pre-deploy
  ** lint:js:watch: for development only -- re-run linting on code changes
  ** lint:js:fix: auto-fix violations of "fixable" ESLint rules. (e.g.: whitespace, quoting scheme, const and let vs var...)