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.
I think the way you have it makes more sense: if there are no [aria-owns] elements then this rule is not relevant. The fact that the element which will end up having an error is not one of these elements is OK.