Well this is my opinion.
I always thought CouchDB offers a great RESTful interface to communicate with their database. CouchDB also offers great multi-master replication, this also includes your local data.
There is a JavaScript implementation of CouchDB's data sync called PouchDB. I always thought it would be a great idea to just use CouchDB as your API layer. Anything your backend needs to do goes through Couch first and get synced to the client through Pouch.
I haven't experimented on the idea yet, but maybe you can take it further.
The IBM bluemix mobile data service sounds like a good match: https://www.ng.bluemix.net/docs/#services/mobiledata/index.h...
"With the Mobile Cloud Services SDK, you can embed a special managed directory in your application. Any files that are stored in this managed directory can be monitored and synchronized. The application can share the contents of this managed directory by connecting to Mobile Data with the same application ID and user ID. By sharing the directory contents, different instances of an application can have synchronized copies of the files"