Proper message queue behavior in pubsub

[minghuaw]

The current pubsub implementation simply discards the message if there is no connected subscriber, which is not how a proper message queue should behave. Another problem is that this is not reflected in the documentation either.

• Fix documentation. Describe the actual behavior
• Retain the message until there is at least one subscriber
  • Should this be configurable? -> This would then break the protocol and thus requires a breaking release

[minghuaw]

#38 is probably caused by this