pluggable url factory

I've implemented support for pluggable URL factories. It should be a fairly low impact change, as the default is to use the current behavior. Should I post a patch here for review before committing? In short, there is now an XWikiURLFactoryService (also pluggable) which is responsible for creating the XWikiUrlFactory for the given mode (XWikiContext.getMode()) . I made XWikiURLFactoryService pluggable as well to allow one to allow one to create the url factories however desired, but its default implementation copies the way they are current constructed. Matt