This moves the first example from:
    https://cloud.google.com/appengine/docs/java/appidentity/
to GitHub.

I also add a simple unit test to at least make sure this code builds &
runs.  Since this sample just wraps:
    ApiProxy.getCurrentEnvironment()
        .getAttributes()
        .get("com.google.appengine.runtime.default_version_hostname"))
there isn't really much more we can test in this example.

I will move the other examples from that page over in later PRs.

Since many users may want to run with multiple projects/versions (dev,
staging, prod) document how to deploy this sample without having to
modify the application-web.xml file.

This flag seems to be ignored by the gcloud plugin and the Managed VMs
environment creates a new version if one isn't provided, anyway.