[act] (1) * Header cells can now themselves have headers.

* Reversed the way the header association algorithm is presented, such
  that it starts from a cell and reports the headers rather than
  generating the list of headers for each cell on a header-by-header
  basis.

* Made it so that if headers="" points to a <td> element, the
  association is now set up, despite being non-conforming.

* Header cells that are automatically associating now no longer stop
  associating when they hit equivalent cells until they have also hit
  a data cell.

* The "col" and "row" scope values now act like the implied auto value
  except that they force the direction.

* Empty header cells no longer get automatically associated.

* Removed the wide header cell heuristic.

* Made headers="" use the same ID discovery mechanism as
  getElementById() (namely, first match), to avoid implementations
  having to support multiple such mechanisms.

* Made the spec define if a header is a column header or a row header
  in the case where scope="" is omitted.

* Made scope=rowgroup and scope=colgroup non-conforming when not in an
  appropriate group.

git-svn-id: http://svn.whatwg.org/webapps@2547 340c8d12-0b0e-0410-8428-c7bf67bfef74