Simple Image Tracking mode doesn't detect URLs automatically #1528

The Simple Tracking Code api could detect the current URL. Indeed, the browser will pass to piwik.php the header Referer, which is the URL of the page loading the Simple Image beacon.

In this use case, we should set URL = REFERER URL.

This will make Simple Tracking Image API more useful when parameters not customized (ie. ebay, myspace, etc.).

(In [2789]) Fixes #1528

• Also adding in product doc about parameters to the Simple Image Tracking code