Modified shout/whisper API

- removed zyre_event_send_shout/whisper which was redundent with
  zyre_shout/whisper
- changed zyre_shout/whisper to take peer/group name as second
  argument, as string, rather than as message
- added zyre_shouts/whispers to send string to peer/group in a
  single step, which is the 90% use case
- modified all test tools to use string sending methods

Notes

- when a method uses class X as first argument that means the method
  lives inside class X, thus zyre_whisper (zyre_t *self,...)