DM remote recipients

(This PR supersedes https://github.com/Libertree/libertree-client-rb/pull/2 )

Updates to req_message .

See also:

https://github.com/Libertree/libertree/pull/8
https://github.com/Libertree/libertree-model-rb/pull/5
https://github.com/Libertree/libertree-backend-rb/pull/5
https://github.com/Libertree/libertree-frontend-ramaze/pull/31