Bridges Dust events to Phoenix.PubSub.

When configured with a :pubsub option, registers a catch-all ** callback on each store that broadcasts every event to the PubSub topic "dust:{store}".

Subscribers subscribe to the store topic and filter in handle_info:

Phoenix.PubSub.subscribe(MyApp.PubSub, "dust:james/blog")

def handle_info({:dust_event, event}, state) do
  # event is a map with :store, :path, :op, :value, etc.
end