Starting work on accessible tables

This is my first attempt to modify unionTables.pl for accessibility purposes. This first commit should give the AlignedRow command the capacity to build the HTML using TH in some places instead of TD. This is my first pull request to openwebwork, so I'm starting small like this to see if I'm doing this right.

I plan to move on to 
 -make the other Row command also be capable of using TH
 -add a caption tag to the tables themselves
 -and lastly add an entirely different CSS-based subroutine for layout tables, which is recommended by accessibility experts. (A layout table is a table where the table environment was really just used for layout purposes, whereas a data table is more of a traditional table with data.)