Piwik: add option to collect statistics with Piwik

Hi Michael,

Please find a patch below which should allow Piwik (http://piwik.org/) stats users to edit config.yaml to enable Piwik data collection similar to Google Analytics configuration.

Not 100% sure on whether I edited the correct config.sample.yml — let me know if it isn't.

Best wishes,

Alex

* app/Config/bootstrap.php: Configure::write for PiwikAnalytics.
* app/View/Elements/google_analytics.ctp: extend if to include Piwik branch.
* app/config.sample.yml: add Piwik sample configuration.