answer to message delivery receipts (XEP-0184)

I recently changed from Xabber to Conversations. I like it very much, but my friends which still use Xabber now don't see the green check marks upon successful delivery when they send me a message.
I found out that Xabber requests message delivery receipts (XEP-0184). So I wrote a quick implementation, that announces the message delivery receipt feature and also sends this receipt, if the sender requested it.