Change ColVis buttons to use UL/LI list for markup

- The previous BUTTON elements appeared to cause an intermitent error -
  see https://github.com/DataTables/ColVis/issues/2. This update uses
  the markup suggested in issue #2, ul > li > label, and then an input
  and span element in the button

- Update CSS to take account of the change in markup

- Adding dataTables.colvis.jqueryui.css file for ThemeRoller integration