Support for _session and cookies in couchbase-lite-ios

We are currently working to get _session support in couchbase-lite. It is probably not ready to go yet, but it is working for us with simple authentication. I know that _session support was explicitly skipped, but I wondered if having code would change that. The code is similar in nature to the erlang (which I used as a basis) but there is not yet support for pbkdf2 passwords. Comments on this are very welcome.

NOTE: This branch has a couple of project fixes in it too so that it can run as a subproject but I don't know that those should be part of this PR.