Full set of global attrs should be allowed on cover figure element.
Updated Schema to use @data-type as the attribute to indicate semantics, rather than @class, so that class can be left for user-defined purposes (e.g., CSS styling).
Modified htmlbook sample to use data-type for semantics.
Modified Alice in Wonderland sample to use data-type for semantics.
Updated specification to reflect the use of @data-type for formal semantic inflection, instead of @class.
Merge pull request #51 from oreillymedia/xmlschema

Updated Schema and samples to use @data-type instead of @class