Upgrade to updated layout

 - Rename models.json to model-config.json
 - Merge "rest/" into "server/"
 - Move "models/" to "common/models/"
 - Nest "lbclient" and "ngapp" under "client" dir
 - and some extra code cleanup

Perhaps the organisation of "client/" directory can be improved, but I'd rather leave that out for another pull request.

The resulting project layout can be viewed here:
https://github.com/strongloop/loopback-example-full-stack/tree/feature/upgrade-to-updated-layout

/to @ritch please review
/cc @raymondfeng @altsang