RESTClient: remove Webclient dependency (#557).

The RESTClient no longer depends on the Webclient.
This makes it more easy do deploy the RESTClient in 3rd-party DM Webclients (like Malte's Twitter-Citymap).

CHANGES

The `config` argument passed to the RESTClient constructor takes another property:
    `on_send_request` -- Optional: the callback invoked before a request is sent (a function). One argument is passed: the request, an object with these properties: `method`, `uri`, `header`, and `data`.

Close #557.