Ports from aria-toolkit:
 - Added check for aria-owns referring to non-existent DOM elements.
 - Added check for the same element having more than one explicit owner.
 - Added new utility method 'axs.utils.getIdReferrers' to facilitate the new tests.
 - Added unit tests for all of the above.
 - Unified 'non-existent' checks for aria-owns and aria-labelledby into a generic 'NonExistentAriaRelatedElement' test.
 - Added new utility method 'axs.utils.getAriaPropertiesByValueType'.
 - Added new utility method 'axs.utils.getSelectorForAriaProperties'.
 - Added unit tests for all of the above.
Completed unification of 'non-existent' checks for aria-owns and aria-labelledby into a generic 'NonExistentAriaRelatedElement' test. This involved renaming files and updating unit tests.
The non-existent test should now cover the following: aria-activedescendant, aria-controls, aria-describedby, aria-flowto, aria-labelledby and aria-owns
 * Ported audit for 'AriaOwnsDescendant' from aria-toolkit.
 * Added unit tests for 'AriaOwnsDescendant'.
 * Gave IE11 some love, all errors fixed and now only one unit test failing.
 * Did a significant refactor on my earlier work to try to better fit the style of the existing codebase.
 * Fixed several random little typos that caught my eye, such as missing semicolons.
Minor tweaks to code style, line length, whitespace, jsdoc etc.
Added unit tests for RequiredAriaAttributeMissing (because adding implicit semantics will affect that functionality).
A couple of other extremely minor formatting tweaks.
Updating as per Alice's feedback.
Updating as per Alice's feedback.
Minor updates as per Alice's feedback: swapping result variables for immediate returns.
Many more tweaks as per feedback.
Added 'aria-busy' and 'aria-hidden' unit tests to 'non-existent-aria-related-element-test' to help demo my question.
fixing minor style issues
more tweaks as per feedback on pull request
style fix
Merge remote-tracking branch 'upstream/master'

Conflicts:
	test/index.html
Merge remote-tracking branch 'upstream/master'

Conflicts:
	test/index.html
Merge pull request #100 from RickSBrown/master

Ports from aria-toolkit and enhancements
Have fixed all the nits. Merging it. Will address some of the TODOs in my next pull request.