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.

Fixes #628