Add selftest related to issue #127 (Steps with trailing colon).
Provide new selftests structure.
Provide description for selftest purpose.
FIX: Undefined step causes scenario.status='failed' (was: passed if undefined step was last step in scenario).
Address corner case when feature file w/o feature is parsed.
Improve code-coverage measuring support.
Issue #117: context.execute_steps() support now steps with multi-line text or table.
FIX issue #135: Avoid leaking globals between step modules. execute_steps(): Minor cleanup/simplification.