ProgressFormatterFactory and PrettFormatterFactory listener trees aligned
General statistics collection and printing moved to the StatisticsListener
Scenario stats collection logic moved to ScenarioStatsListener
Hook stats collecting logic moved to HookStatsListener
ExerciseListener is renamed to StepStatsListener
StepStatsListener docBlock updated to reflect class responsibility change
Merge pull request #463 from Behat/optimization/exercise-listener-simplification

Exercise listener simplification