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/

[1]: https://cloudant.com/

[2]: https://github.com/colinskow/superlogin

[3]: https://github.com/cloudant/CDTDatastore

[4]: https://github.com/cloudant/sync-android