Piwik: add option to collect statistics with Piwik

* 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.
Merge pull request #85 from PTFS-Europe/piwik

Piwik: add option to collect statistics with Piwik