Added Event Bus for pipeline interconnection messages and integrated it with ValveFilter

[bwplotka]

Notes about the bus (from doc)

• There is only one EventBus (as singleton) in serenity.so
• EventBus and subscriber is a separate libprocess actor (currently ProtobufProcess)
• API inpired by: http://doc.akka.io/docs/akka/snapshot/scala/event-bus.html
• Extremely easy to configure connection between components. Flow:
• Define message in proto including Envelope. (libprocess limit)

message SomeEventEnvelope {
    message SomeEvent {
        optional bool first_event_param = 1;
        optional bool second_event_param = 2;
        (...)
     }
     optional SomeEvent message = 1;
 }

1. Create subscriber. Derive from ProtobufProcess.
2. Install endpoint and handler for SomeEventEnvelope in Subscriber.
3. Register subscriber: EventBus::subscribe(subscriberPID)
4. When event is ready just do EventBus::publish(msg)
   • Publisher does not need to be an libprocess actor!

Based on Szymon's bus: https://github.com/mesosphere/serenity/pull/91

This PR includes: https://github.com/mesosphere/serenity/pull/93

Also this PR includes integration with ValveFilter.

[bwplotka]

Updated version: https://github.com/mesosphere/serenity/pull/98