Changing roles to be more compatible with screen readers

Changed the roles of the header table, data table, and footer table to
be "presentation" and changed the parent element of these tables to have
role="grid".

Also changed all usages of the role "rowheader" to "row" because it was
being used incorrectly. See
http://www.w3.org/TR/wai-aria/roles#rowheader

These changes will provide better screen reader support. Tested with
JAWS screen reader.