Update StepProcessor

* Now takes an XMLElem and a Map[String, String] as inputs and returns a Stream[String] with the natural language step descriptions.
* Solves #3.
Update StepProcessor and test

* Function processSources now takes an Option[Map[_]]. If it is None there is no need to try to find and replace words from it.
* Function processSources now returns String instead of Stream[String]. It should be the task of the StepProcessor module to provide the correct content that can be written to a file without any further modifications.
* Adapted StepProcessorSpec to the changes with stubs.