Webclient: process directives from plugins (#556).

The Webclient (actually the RESTClient) automatically processes directives sent by a plugin service.
So a plugin resource method can return directives now. They are processed at client-side automatically.
Existing plugins require no changes.

CHANGES

3rd-party DM webclients:

The RESTClient constructor method has changed. It takes one optional `config` object now as the argument.
For the moment the `config` object expects one (optional) property:
    `process_directives` -- a function that processes the directives returned by the server.

The former `core_service_uri` RESTClient constructor paramter is gone. As the core service uri `/core` is now hardcoded.

See #556.