thumbCaptions setting creates invalid html

[mattdyer]

There is a thumbCaptions setting that, if set to true, looks for a data-thumbcaption attribute on the slides and uses that to add a caption to each thumbnail. This only activates if controlNav is set to "thumbnails". When it adds the caption, it appends a span to the img elements that are the thumbnails. Img elements can't contain other elements, so the captions never display.