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.