Mavenized project per JDM (see links)

Mavenized project per JDM. 

Allows for CI, greater testability.

`/war` directory removed.

References:
https://developers.google.com/appengine/docs/java/tools/maven#configuring_pomxml_settings_for_google_app_engine

https://github.com/GoogleCloudPlatform/appengine-guestbook-java