Author: Damyon Wiese
MDL-44116 Atto Accessbility checker: Additional sniffs.
http://git.moodle.org/gw?p=moodle.git;a=commit;h=e5b5b5be4db08801409355b94ae9c602cdf60d81
http://github.com/moodle/moodle/commit/e5b5b5be4db08801409355b94ae9c602cdf60d81
+ 2.8 en [emptytext,atto_accessibilitychecker]
+ 2.8 en [needsmoreheadings,atto_accessibilitychecker]
+ 2.8 en [tablesmissingcaption,atto_accessibilitychecker]
+ 2.8 en [tablesmissingheaders,atto_accessibilitychecker]
+ 2.8 en [tableswithmergedcells,atto_accessibilitychecker]