app: update `url` on `listening` event

Most applications report the URL when started (at least the apps we
are scaffolding using loopback-workspace). Constructing the URL in the
loopback core allows us to simplify the templates and reduce the amount
of repeated code.
Merge pull request #357 from strongloop/feature/set-url-on-app-listening

app: update `url` on `listening` event