Add first and last classes to table header cells

Issue Summary

In the initial d7ux theme issue, I've implemented table borders as it was possible with the current markup and classes, which caused them not to match the d7ux mockups. The mockups call for border on the left and right end of the table header to not exist. Since Drupal does not provide any markup to identify first and last table header cells in the table header, there was no way to hang styles on these elements. To fix this issue, we need to add first and last classes to table headers just like we do with lists. While the slate theme can just copy the theme_table() monster function, this simple modification is possibly useful to other themers too, so I'm proposing it for the core theme_table instead.