~samwhited/xmpp

ref: 52be89df867be9fc4e8ba83b0c3880887a6485ba xmpp/ibr2 d---------
ibr2: Always send a response
ibr2: Implement OOB challenges
ibr2: Add initial negotiation implementation
ibr2: Add API stability disclaimer to docs
ibr2: Fix broken string formatting in tests
Add an experimental IBR2 API

(the implementation is not actually done yet)