List widget now implement the WIA ARIA Grid pattern defined in
http://www.w3.org/TR/2013/WD-wai-aria-practices-20130307/#grid

This version solves issue #55
(default renderer labels should not be separately focusable).

This version also allows using actionable HTML inputs in items renderer
(see the list samples).
Update Button to use bootstrap variable.less
Update ProgressIndicator to use bootstrap variable.less
Update Slider to use bootstrap variable.less