better GAE support based on latest restx

including GAE default authentication support

If you prefer to use RESTX authentication mechanism, feel free to discard this PR, or include it on a branch.

This example is interesting because it demonstrates how to use standard Servlet security with RESTX, and even customize the principal returned to Restx, so that if the user has GAE `admin` access it will be granted access to RESTX console (`restx-admin` role).

This could easily be extended to other standard servlet security integration, eg spring security.

Note that I had to remove the requirement for role 'HELLO' on message endpoint, because principal will either have no role or only `RESTX-ADMIN` role.

I also to fix the path of settings loading, because of a bug in app new generation in the restx version you used (which should now be fixed on master).