Support delayed delivery (XEP-0203, XEP-0091).

I added support for the <delay/> element, which is required to get the correct time when e.g. receiving history form the chatroom.  There is a new and a legacy standard; the ejabberd version I'm using myself uses legacy, so I had to add that, as well.

I did not attempt to parse the dates, but instead pass the stamp string verbatim to the client callback.  If it's okay to depend on the Unix module or some external calendar library, I can change the commit to parse the dates into an float (seconds since Epoch).  I think that makes the "delay_legacy" field unnecessary.

(A dependency on some date providing module may also be relevant for formatting the ~since parameter for MUC, which should be an XMPP date string.)