Add BlackHoleActor & EchoActor to TestKit

BlackHoleActor is an actor that silently accepts any messages to it.
EchoActor forwards messages it receives to the TestKit's TestActor as
well as to the sender (can opt out) unless the sender is the TestActor.
Merge pull request #341 from HCanber/testactors

Add BlackHoleActor & EchoActor to TestKit

BlackHoleActor is an actor that silently accepts any messages to it.

EchoActor forwards messages it receives to the TestKit's TestActor as
well as to the sender (can opt out) unless the sender is the TestActor.