Category export improvements

Last week, the categoryi_export module had some significant improvements made to it (sorry for the late announcement - forgot to post here when it happened). There is now a new hook_category_export(), which allows other modules to tell the category system about their export formats. Export modules that implement this API can now have their export format toggled on or off on a per-containeri basis. Category_export is currently the only module that implements this new API.

Book export modules, which allow the export of categories through the book wrapperi, do not implement this API. However, they can still be used the same as before: they are simply not able to be toggled per-container; they must be enabled for all containers and categories on a site.