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

Mirrors the API of Mink::getSession but does not auto-start the
session to make it possible to check whether the browser has already
executed a request.

Fixes Behat/Mink#273
Merge pull request #326 from acoulton/feat/is-session-started

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