FIX issue #162: Unnecessary ContextMaskWarnings when assert fails or exception is raised.
Verified that issues #28, #33 are fixed (related to installation on Windows).
Finalize version: 1.2.3a19
BUMP-VERSION: 1.2.3a20, for next development steps (increment).
FIX issue #164: StepRegistry.find_match() extends registered step_type lists.
NEW: StepsFormatter that provides an overview of existing step implementations.