New: CSS - Add .selected option to CSS for selected rows

- It is quite common to have a row selection styling in DataTables, so
  it makes sense to built this ability directly into the core CSS. It
  doesn't add too much extra weight, but it would be a huge pain for
  developers to do themselves and also it means I can reuse it in
  TableTools / Editor.
Dev fix: Additional to the content-box fix form the other day - use
jQuery since browser specific styles can override the W3C option. jQuery
deals with it for us.