Support userreference fields

Similar to CCK's text and number fields, it would make a lot of sense to support other fields. The "userreference" field type ships with CCK itself, and therefore pretty important to support. The getter/setter callbacks should retrieve/write node objects, while for the import/export callbacks it's probably sufficient to simply return the uid itself.

("Just the uid" because we definitely don't want to deal with differing uids between sites... that's way too much complexity, and really needs to be solved by other means, e.g. unique identifiers, user name comparisons, or assuming that both sites have similar user ids for the same users.)