HtmlCssHelper: avoid writing string "null" in CSS
XHTMLImporter: revert change to interface, and do setBookmarkIdNext via reflection for 3.2.1 release.
Remove Rel value getter/setter
misc minor
minor documentation updates
Use jcl-over-slf4j instead of FOP's commons-logging
add release target
handle NPE
Reinstate access to partName field, for better backwards compatibility
prep for 3.2.1 release