Added sendDraft and refactored to use it instead of send.

All interactions with pond.Message from cli.go and gui.go now take place in
network.go.  sendDraft func isolates updating the ratchet.

I believe this should simplify sending messages to multiple recipiants,
and reduce the chances of making mistakes when writing future UIs.

This should also fix a minor hickup in the GUI where, if the user begins
a reply, but deletes the inbox message before replying, then the reply
loses its status as a reply.