Adds pub/sub of incoming Pss messages using event.Feed,
and a websockets interface for pss send and notifications
through persistent connection using rpc.notifier.
Websocket RPC are started automatically on the swarm executable
if the --pss flag is present. If more instances are to be run,
alternate ws ports can be specified with the --pssport flag.
Current implementation also includes a shallow builtin ping/pong handler
with topic "pss" and version 1, to simplify POCs and testing against pss.
This will eventually be removed, so no code should be made dependent on it.
