New 'text_sortable' field

Generates a label + input:text with a dragging image to reorder the
fields (look at http://img.saika.li/M21i you'll understand).
The fields must be added using the 'options' => array('key' => 'value').
I hard-coded the width of the label (6.5em), there might be a better
way to do this.
Added Chosen sprite

http://harvesthq.github.com/chosen/