Fix MathML namespace in crawler.

Summary: Crawler MathML elements need to declare the MathML namespace.

Test Plan:
Run xhtml2harvests on xhtml file, load harvest into daemon and
query using an XmlHttpRequest. This will return a valid responseXML.

Reviewers: m.iancu

Reviewed By: m.iancu

Differential Revision: http://phab.code4fun.de/D22