Process MathML elements

This fixes #11:
* copies presentation MathML from nxml files
* removes the namespace
* add the new custom type pmml to the Mediawiki Math tag
  to give the MediaWiki:Math extension a chance to treat
  the input as mathml input rather than tex(vc) input
Merge commit '093b9584272205b41b623a2133807394e24b8615'