Added GhostDriver/Phantomjs support

This add Phantomjs direct support (instead of using a workaround with selenium2 confs).
On top of that, you can now set Custom Headers and Basic auth throught behat.yml (and other options available with GhostDriver).

I created the MinkGhostDriver implementation from Selenium2 driver, available here (and on Packagist) : https://github.com/Adezandee/MinkGhostDriver