Ports from aria-toolkit and enhancements

Added audit for multiple explicit owners.
Added audit for aria-owns DOM descendant.

Enhanced (and renamed) "non-existent-labelledby" audit to cover all id reference types: 
 * aria-activedescendant
 * aria-controls
 * aria-describedby
 * aria-flowto
 * aria-labelledby
 * aria-owns

Added a few utility methods to support some of the new functionality.
Added IE11 support.
Added unit tests for RequiredAriaAttributeMissing audit.
Unit tests for all changes.
This raises a question I have been meaning to float. Should the severity take into account whether the element is aria-busy or hidden?

This would probably not apply to all audits (like the abstract role check for example) but missing 'required owned elements', 'required states an properties' and even this IDREF audit could all possibly be affected?

Should we be checking hidden elements at all? Probably yes but should they be treated the same?

Also, is this the right place for me to raise these sorts of questions or is there another forum?