Add AngularJS API docs for built-in 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.
services: make ngdoc comments jsdoc compatible