Expose Scenario id to step definitions

Recently on the [mailing list](https://groups.google.com/d/msg/cukes/b91lZ0WwuBE/dWmq6WAFgrsJ) the need to access the Scenario name and ideally the concatenation of Feature/Scenario Outline/Example from step definitions, which actually is the Id of the Gherkin Scenario. Now when the Scenario name has been exposed to the step definitions through the cucumber.api.Scenario interface (#671), it makes sense to also expose the Scenario id through that interface.

Fixes #715.