app.start() calls app.listen()

Use the new `app.listen()` method from the loopback core to start
the server.
Refactor explorer integration, add "started" event

Modify `app.start` to emit "started" event.

Move all code related to the setup of loopback-explorer at one place,
use the new event "started" to report the URL of the explorer.
Merge pull request #37 from strongloop/feature/app-listen

Use `app.listen`, add "started" event, simplify explorer integration