XEP-0184 rewrite

Here is an extended patchset for XEP-0184, according to the submission rules:

 * refactored the Provider class(es) to be inner classes of the respective packet types
 * added DeliveryReceiptManager to handle management, enabling and disabling of automatic replies
 * added test-cases

As last time, if you like the patches, I'll squash them after review and provide one single commit. Just tell me if that commit should also contain the XEP-0184 commit you already have in the master tree, so you can push one single patch to upstream.