Update `user-streams` branch to use AFNetworking 2

After having a quick chat to @kolinkrewinkle, we thought it might be best to  open this pull request and start discussion of how to proceed. This doesn't merge cleanly — I'll be looking at that, but otherwise it would be good to get this merged into the user-streams branch and get that tidied up for release (I can see there are probably commits on `master` that need to be brought across on top of this as well).