Visitor interface implementation.

Hook which give possibility to manipulate Visitor object instance. I've used same convention which is implemented in Tracker class. (https://github.com/piwik/piwik/blob/master/core/Tracker.php#L631) This hook could be used to overwrite some fields in Live plugin API methods. Interface is simply as possible. Only two methods should be visible outside of Visitor class. (in my, and my phpstorm opinion)