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.
Sorry, I should have mentioned this in the last round: TODO comments should take the form
// TODO(yourname): <what should be done>.
http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml?showone=Implementation_Comments#TODO_Comments