I'd suggest using PouchDB[0] if on a web client, and Cloudant [1] as the backend. Spin up a simple heroku app using Superlogin[2] and you have full Auth, an offline database, sync and now the possibility of conflicts resolution.
If developing on mobile clients just use Cloudant Sync for iOS[3] or Android[4].
[0]: http://pouchdb.com/
[2]: https://github.com/colinskow/superlogin