Refactor: Move server.js into app.js

As we prepare for testing the server,
move everything into a new file.
Refactor: Server: Extract method: startServer
App: Do not start unless asked

We now have an app module that is
ready to be tested, as we can load
it without starting an http server,
and we have exposed the app object.
Specify npm start script: node server.js

Since we have two JavaScript files in our
app folder, and it's not obvious which
one to run, specify it via the start
script in our package.json.