Update boot() to serve the app asynchronously

See #170 
This should be implemented by loopback-boot, please remove L19-L20.

It would be nice to add a unit-test for this too, see https://github.com/strongloop/loopback-workspace/blob/master/test/end-to-end.js.

The test can do this for example:
 - scaffold a new app
 - add async boot script
 - require server/server.js
 - check that there is a flag indicating async boot process and that the event `ready` is eventually emitted