Generate usage of JGiven stages in test methods.

[ingokuba]

1. generate @ClassRule JGivenClassRule featureRule
2. generate @Rule JGivenMethodRule scenarioRule
3. generate @ScenarioStage annotated stages for Given, When, Then (see Generate JGiven stages from scenario steps. #24 )
4. generate private convenience methods given(), when(), then() wrapping the given, when, then methods of initialized stages
5. generate calls of corresponding jgiven stages according to scenario steps