Fully merge of Behat/MinkWUnitDriver

Looking at Behat/Symfony2Extension, I decided to fully merge the newly created Behat/MinkWUnitDriver into this extension.
Now, the mink driver wunit is loaded either if explicitly requested or Behat/MinkExtension and Behat/MinkBrowserKitDriver is available.
For using it, one still has to configure the behat.yml properly, setting default_session to wunit.