Thumbnails controlNav update

When controlNav has been set to 'thumbnails' until now, for no 
reason the images in the nav has been used as selectors, so I've wrapped
them into a link. This is better, because You now can inject some hover
into the thumbnails control nav  without breaking it.

See http://demo.dotwired.de/necon/ and hover the thumbnails to see what I mean by "breaking" it. As soon as you hover the "View" circle, You cannot change the slide any more.

PS.: I've set the li as a new .flex-active selector, you may have other thoughts about this.