A Singleton IModel implementation.

In PureMVC, the Model class provides access to model objects (Proxies) by named lookup.

The Model assumes these responsibilities:

• Maintain a cache of IProxy instances.
• Provide methods for registering, retrieving, and removing IProxy instances.

Your application must register IProxy instances with the Model. Typically, you use an ICommand to create and register IProxy instances once the Facade has initialized the Core actors.