Working Testing chapter.

modified:   project/AkkaBookBuild.scala
modified:   src/main/scala/zzz/akka/avionics/Altimeter.scala
new file:   src/main/scala/zzz/akka/avionics/Avionics.scala
new file:   src/main/scala/zzz/akka/avionics/EventSource.scala
modified:   src/main/scala/zzz/akka/avionics/Plane.scala
new file:   src/test/scala/zzz/akka/EventSourceSpec.scala