Remove super() call to initialize Application as Registry

No longer necessary since the underlying _components dict is now a reified
property.
Rework resource configuration

Overall, consolidate and simplify. Move away from "representation info"
terminology in favor of "resource config" since that better represents
what `@config` is actually for (which is the same reason it was
previously renamed from `@represent`).

- Add resource.config.Config class to validate and contain resource
  configuration.
- Rename app.add_representation_info_field() to add_config_field().
- Remove app.get_default_representation_info(); replace with simply
  creating an instance of Config.
- Remove app.get_representation_info(); replace with creating a
  Config instance via the Config.for_resource() factory.
- Defaults for `@config` args can now be factories; i.e., if a callable
  is passed as the default, that callable will be called to get the
  default.
- Remove confusing usage of NOT_SET.