New Service: Piwik

Hello,

I've added [Piwik](http://piwik.org/) tracking support to the library. I ran the unit tests against my own Piwik instance, however I don't have a open instance that I can publicly share and include in the commits. As of such when running the tests you'll have to point the url and id to a valid piwik instance (test/utils.js line 147).