taglib <webanalytics:tracker /> learn the 'useAssets' property

This property provide the ability to enable (by default) or disable the use of <dandelion:assets /> to render the assets.
It's useful when you just need to use <webanalytics:tracker /> in a page.

If useAssets is false, <webanalytics:tracker /> perform the render of webanalytics script by himself.

closes dandelion/issues#126