Pass world arguments as options, not args

There are more options but only some will be set often. This allows
better way how to override the defaults.
ExecutionPlan#state
Move state symbol conversion to ExecutionPlan#state=
Test for making sure ExecutionPlan#state is :running when execution in progress
Add fully parallel executor
Use RoundRobin in Pool

typo fixed
some method renaming
track failed steps in Parallel executor