Allow to retrieve scope names for a given active record class

A while back, it was possible to loop through the scopes of a given model and retrieve a Proc for each scope.

Today, scopes are methods but we can't list them.

It would be great to list at least the name of the registered scopes for a given model. This PR covers this basic functionality.

Maybe the naming is not perfect, feel free to propose another name for the attribute.

Thanks