Rewrite the feature file for flow_manager, delete a comment outed code, and disable module's debug comment.

modified:   features/flow_manager.feature 	: Rewrite it to be more understandable.
modified:   ruby/trema/flow-manager-module.c 	: Delete a comment outed code and disable module's debug comment.
modified:   ruby/trema/flow-manager-path.c	: Add a inline comment.