Added Mink::isSessionStarted to check if a request has been made

@everzet this is the pull request for Behat/Mink#273 - I named the method isSessionStarted which I thought was clearer than isStarted in that context. I've mirrored the API (and tests) for getSession, and therefore refactored the code that checks the session exists into a shared method.

I tried to follow your coding style and testcase structure - do let me know if there's anything you want to see different.

Thanks.