Discovery: Add special endpoint to get all the resources.

Broken down to version and sub version. To dig for more information one would need to make an OPTIONS call to the particular resource (with the version options).

This and https://github.com/Gizra/restful/issues/197 are the _basics_ for discovery features that I can see. With this in place I can see a `restful_apiary` contrib module that generates automatic up-to date documentation.