Issue #241 - add ability to use custom cookie login

- Add a flag to either use custom cookie or fb login
- Call new replication api to set cookie (added in issue 241)
- Add new observable for cases when the replication stops due to auth errors.

https://github.com/couchbase/couchbase-lite-android/issues/241
Removing user id was not working, since apply() wasn't called

https://github.com/couchbase/couchbase-lite-android/issues/269
Issue #269 - use persistent cookie

https://github.com/couchbase/couchbase-lite-android/issues/269
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