Moved the callong of on_send_message.

  This is now only present in one location, and we also call
 on_senT_message afterwards.

  Part of #131.
  get_signature returns an UTF8 string.
  Avoid duplicating the "Send mail: y/n/a" code.

  This is now moved into `should_send` so that it appears once
 instead of three times.

  This closes #131.