Fix: Accessiblity - Add `role=row` attribute to table rows

- Feedback from James Craig on improving the accessiblity of DataTables.
  Since the `role=grid` option is added to the DataTables table, we
  should also add `role=row` to the rows in the table so the browsers
  can see them correctly. As James notes, it is debatable if this is
  needed or not but it is required in current Chrome and Webkit.

- Webkit do now allow roles to be optional in the nighties - see:
  https://bugs.webkit.org/show_bug.cgi?id=131819 . Once Apple deploy
  this in a Safari update, and Chrome also deploy support, I think this
  should be removed from DataTables core, since it will no longer be
  needed in the latest browsers, and manipulating the DOM can hit
  performance a bit.