Add featureStack to handle cleanup after tests.

 - FeatureStack.rb contains lambda methods to remove resources
 - should be stacked into @featureStack on every step creating a state change.
 - e.g. @featureStack.push(bookCreated(@context[:book_id]))

Will be run in reversed order after feature run, so this demands features create resources in specific order. Perhaps ordering logic should be moved into FeatureStack.

Also some code cleanup in steps