Added Scenario step context

Hi I have added support for a basic ScenarioStepContext which holds the current StepDefinitionType and step text. This can then be used in BeforeStep/AfterStep methods so you can log the name of the step which is currently running or the name of the step at which the test failed.

I have seen several requests for such functionality like [here](http://stackoverflow.com/questions/16335419/specflow-query-i-want-to-use-beforestep-to-put-a-line-into-a-log-report-and-so) and [here](https://groups.google.com/forum/#!topic/specflow/aL5v6mLdgEg).

There was already a StepContext class and initially I started to add to this, but the purpose of this class is not clear and once I started modifying it it quickly got quite invasive. So I decided to create a new class for this extra data to hang off, hence the slightly non intuitive name of ScenarioStepContext.