Support async boot scripts OOTB

[bajtos]

We need to rework server/server.js template to support async boot scripts.

boot(app, __dirname, function(err) {
  if (err) throw err;
  // start the server if `$ node server.js`
  if (require.main === module) {
    app.start();
  });

In order to support the scenario when the app is loaded via require but not started, e.g. in loopback-sdk-angular-cli, we need to modify loopback-boot to emit an event when the boot process is finished, e.g. app.emit('ready').

Tasks

• Add "booting" flag and emit "booted" event loopback-boot#77 modify loopback-boot to emit ready event (or similar)
• Boot the app asynchronously #172 rework server/server.js template to defer app.start until boot(app) is finished
• Update create sample data script to be async generator-loopback#74 fix slc loopback:example to use async boot script for generating models, see example/templates/server/boot/create-sample-data.js

[bajtos]

Reassigning back to@superkhau, there are three tasks listed in the description of this issue.