fix nogui build
No need for dev bool in noGUIClient.
Switch to late signing.

Previously, messages were signed for delivery (with the group signature)
as soon as they were enqueued. This resulted in a complex dance when
receiving a group update because messages in the outgoing queue needed
to be resigned.

This change switches to late signing: messages are only signed just
before transmission to the destination server.
Merge branch 'master' of https://github.com/agl/pond