Improve the namespace fixup in `px:html-load`

 * the namespace-cleaning XSLT is systematically applied
 * HTML in no namespace is assigbed the default HTML namespace
 * MathML content is assigned the MathML namespace
 * SVG content is assigned the MathML namespace
 * added XSpec tests

Fixes [issue 349](https://code.google.com/p/daisy-pipeline/issues/detail?id=349).