Support Akka actor tracing

[takezoe]

This pull request makes possible to trace local actor calls from Play. Here are example of traceable actors:

import jp.co.bizreach.trace.akka.actor.ActorTraceSupport._

// Message extends TraceMessage and take TraceData
case class HelloActorMessage(message: String)
  (implicit val traceData: ActorTraceData) extends TraceMessage

// Actors extend TraceableActor and inject ZipkinTraceServiceLike
class HelloActor @Inject()(@Named("child-hello-actor") child: ActorRef, 
                           val tracer: ZipkinTraceServiceLike) extends TraceableActor {
  def receive = {
    case m: HelloActorMessage => {
      println(m.message)
      // Decorate actorRef with TraceableActorRef() 
      TraceableActorRef(child) ! HelloActorMessage("This is a child actor call!")
    }
  }
}

// It's possible to trace nested actor calls
class ChildHelloActor @Inject()(val tracer: ZipkinTraceServiceLike) extends TraceableActor {
  def receive = {
    case m: HelloActorMessage => {
      println(m.message)
    }
  }
}

Result:

For remote actors, a message class must extend RemoteTraceMessage instead of TraceMessage.

case class HelloActorMessage(message: String)
  (implicit val traceData: RemoteActorTraceData) extends RemoteTraceMessage

In this case, the span data is converted to java.util.HashMap[String, String] automatically to serialize.