Add multi favicon support

http://www.netmagazine.com/features/create-perfect-favicon 
in this article, it tells how to support large icon for different devices and browsers, using the <link> element: <link rel="icon" type="image/png" href="/pathto/favicon.png" />

I modified base_helper.tmpl to support this, adn post it here to see whether this is usefull for others.