connector: load loopback models from filesystem

This patch presents a short-term solution to load definitions of loopback built-in models. It is intentionally not implementing full support for components, because that requires much more effort (see #159).

 - Add a new internal property `ModelDefinition.readonly` to signal whether the definition can be edited by users.
 - Modify connector to search `node_modules/loopback` for model definitions.
 - Fix `ConfigFile` methods to correctly support subclassing.
 - Handle shorthand property definition

Part of https://github.com/strongloop-internal/scrum-loopback/issues/34.
Fixes https://github.com/strongloop/loopback-workspace/issues/161

The task for implementing proper support for components: #159

/to @ritch please review