Add API for Client publish-subscriber code

It uses the Topic as unit to subscribe to
Add simple implementation for TopicSubscribers

It uses the TopicForwarder as implementation.
Also made Topic.apply thread safe, because it will now be called from
multiple threads
Add dependencies to client build.sbt for testing
Add Test for the client-sided Topic systems

Tests the Topic creator and the TopicForwarder/ForwarderActor
Fix problems found from testing

Using the default for a mutable map returned in something kind of
immutable