send along post/comment UID

A post or comment UID is really just a PubSub IRI as defined
in RFC5122; examples are given in
[XEP-0060](http://xmpp.org/extensions/xep-0060.html#impl-uri).

Here's an example for a post with local id 1234 on oak.elephly.net:

    xmpp:oak.elephly.net?;node=/posts;item=1234

The UID obsoletes the pair (origin, post_id).
add a TODO note about future implementation of references
comments reference post UID in thr:in-reply-to tag