implement step to check json subset containment

[steos]

If you have a large json response and you really only care about some subset of it it is very inconvenient to have to compare the complete response so I implemented this simple subset check that let's you recursively compare a json dataset.

[stof]

I'm not in favor of adding more steps to this old context class. We are currently in the process of rewriting the WebApiContext as a proper extension for Behat 3, and we plan to provide more assertion capabilities in the new version. As we are not done yet with these assertions, I prefer avoiding adding new steps to the old context, which would then have to be implemented in the new package while they might not fit the new system.

I will keep this PR opened until we progress a bit more on the new extension