Math not parsed in some docx files

[jgm]

See this discussion on pandoc-discuss.

The docx reader currently seems to expect all the namespace prefixes to be declared on the root document element, but in some docx files they are not -- e.g. this docx has a tag

<m:oMath xmlns:m="http://schemas.openxmlformats.org/officeDocument/2006/math">

where 'm' is not declared on the document element.

We need to be able to recognize math when it is inserted this way.

Side note: the docx reader also seems to expect that m will always be the prefix for math, etc. If we're making this assumption, why do we even bother looking up namespace declarations? Perhaps we shouldn't make the assumption (though it seems solid for every Word file I've seen). Alternatively we could work with the URIs themselves, but then we might run into the problem of different versions (e.g., 2006 vs 2011).

@jkr can you have a look and see what you think should be done?