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 like these JSDoc comments, but these methods are doc'd (poorly) in https://github.com/GoogleChrome/accessibility-developer-tools/blob/master/src/js/AuditRule.js - perhaps these explanations could live there (in the axs.AuditRule constructor)? What do you think? It's mildly annoying that there's not really a concept of an 'interface' in JavaScript...