[WIP] Add AnuglarJS 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.

Add docs.json configured to run the new script and use the generated
services file as a source for strong-docs.

This is a work in progress, strong-docs does not parse ngdoc comments correctly at the moment.

/cc: @ritch @crandmck