For PR|PER, check that the SOTD contains a link to wbs

That PR looks for a link to wbs in the SOTD as suggested in issue #6 . The link could have different forms:
- https://www.w3.org/2002/09/wbs/33280/ (see http://www.w3.org/TR/2014/PR-wai-aria-20140206/)
- http://www.w3.org/2002/09/wbs/myQuestionnaires (see http://www.w3.org/TR/2014/PR-xqueryx-30-20140130/)