Implement EventFiringWebDriver and EventFiringWebElement

See comment on https://github.com/ashleydw/php-webdriver/commit/5355783283f20cd7e84878f13a4df5cc017d7034 for usage.

This is based around http://selenium.googlecode.com/git/docs/api/java/org/openqa/selenium/support/events/EventFiringWebDriver.html