JBEHAVE-1041:  Calculate the story duration from the start of the execution and not the submission of the EnqueuedStory.

Conflicts:
	jbehave-core/src/main/java/org/jbehave/core/embedder/StoryManager.java
	jbehave-core/src/test/java/org/jbehave/core/embedder/EmbedderBehaviour.java