Singleton resources

In this application, we will refer to resources as objects that are related to the URI contained in the API server. This means that to manage the /products URI we are going to have a ProductResource object that will manage the CRUD operations for this URI.

We will create this object as a singleton to guarantee that we have just one object managing each URI in our application. For more information on singleton, refer to http://en.wikipedia.org/wiki/Singleton_pattern.