Document & test XSLT

[rlskoeser]

The XSLT is not very well documented; some of the files have no comments at all even indicating what the stylesheet is for. Consider unit tests for the xslt, since in a sense this is where the bulk of the API logic is, transforming the data. Also look into tools for automated validation of the non-xml outputs generated by the XSLT. (jsonlint, csvlint, rdf parsers?)