Improve persistance API

Any XMLRessource can persist after the agent is shut down. Just set _persist to
True and implement two methods:

    class Ressource(XMLRessource):
        _persist = True

        def _persist_primitive(self):
            """On stop return state primitive that will be saved."""
            return ...

        def _persist_load_primitive(self, primitive):
            """On start logic that restore ressource state from the primitive."""
            ...

Running the agent with -s will _not_ save ressources states on exit.
Running the agent with -l will _not_ load ressources states on start.