test(core/lib/Theme.js): test for schema-valid theme.json

Initial support for validating a theme.json file.  Uses node package "jsonschema" to define a schema for theme.json and test validity of instances.  Report failures as hard errors.
fix(admin module): no crash on empty theme

If a theme does not have a JSON file or it is not readable to the
point where it has a type, then do not fail the admin module.
test(app): debug messages for app.stack building

Log app.stack content in debug mode.