trackPageviewParam option added

Option to add additional parameters to _trackPageview. This allows
support of custom 404 error tracking like the one described here:
http://www.bluefountainmedia.com/blog/track-404-errors-in-google-analytics/

I also changed the logic for the beforeRender callback example in the readme to check if the component is enabled or not. That way an 'enabled' configuration property can be added to config/local.php to disable analytics when running on a local test server.