Add support for retrieving PubSub Node affiliations

This also marks the starting point for extending the PubSub API to allow
additional packet extensions to be added to the request. This is for
example useful if one wants to limit the result with "Result Set
Management (XEP-59)".

Fixes SMACK-580.
Fix typo: s/no_acceptable/not_acceptable

in XMPPError.Condition
Reply correct error code on XEP-65 Stream rejection

this was previously erroneously changed with
197548b548c0436108cb20f33859172a70d0bf8f from 'forbidden' to
'not-acceptable'. We now change it back to 'forbidden'.

Also delete FileTransferNegotiator.rejectStream(StreamInitation) because
it is dead code and add some comments and javadoc.
Improve DeliveryReceipt code, add javadoc

Also make Packet.getExtension() use Generics