Destruction Method For Entities and Associations

Add destruction methods to Entity and AssociationCollection that will destroy the entity from the backend.

• Entity.destroy() will create and execute a destroy operation on the entity's service adaptor. If the entity belongs to an association, the entity should no longer belong to that association after it's been destroyed.
• AssociationCollection.destroy(entity) will create and execute a destroy operation on the given entity. If the association does not contain the entity, an empty operation should be executed.