Command runner

This PR will solve #57 

@mnapoli I still need to change the `exec` call in `\Couscous\Step\Scripts\ExecuteScripts::executeScript` could you help with that please?
from our discussion on gitter it might not have been clear but we follow PSR-1 and 2 for the source, we only use `snake_case` for names of test methods (an example of that is mentioned in this video: https://www.youtube.com/watch?v=1_dpOZmKXBw)

So here `$return_value` should be `$returnValue` (and same applies for the rest of the PR)