Enable strong-doc generated API docs for built-in loopback models

 - Add `apidocs/describe-builtin-models.js` - a script that sets up a loopback app with all built-in models attached and generates the AngularJS services for this app. The generated ngdoc comments are modified to make them compatible with strong-docs' parser.

 - Add `docs.json` configured to run the new script and use the generated
services file as a source for strong-docs.

- Fix `lib/services.template.js` to make ngdoc comments compatible with strong-docs' parser.

A part of https://github.com/strongloop/loopback-sdk-angular/issues/87

/to @ritch please review