Piwik tracking support

Adds tracking code (both javascript and fallback image in noscript) to analytics.html below Google analytics, if `PIWIK_URL` and `PIWIK_SITE_ID` is set.

`PIWIK_URL` needs to be the URL to the Piwik instance without 'http://'.

(Merging my changes to master so I can use them from a submodule without having to figure out how branches and submodules work together. And I get an opportunity to test pull requests! :-)