connector: load loopback models from filesystem

 - 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.

Implements https://github.com/strongloop-internal/scrum-loopback/issues/34.

The task for implementing proper support for components: #159

/to @ritch please review