Added response message case class
Updated tests for EchoPlugin and PongPlugin

EchoPlugin should now return Response message for private messages.
PongPlugin should ignore Response messages.
EchoPlugin now returns Response message instead of PrivateMessage
ChannelActor refactored to StreamWriter and StreamReader
ProcessingActor changed to MessageProcessor
Added master actor.

Master actor is responsible for redirecting messages.
Actors were moved to separate package
Merge pull request #4 from DrunkenPandaFans/akka

Akka branched merged to master