CouchDB is the way to go. Now I am working on a similar project. iOS, Android, web client - all of them need synchronization. CouchDB has fantastic HTTP replication support. This is a thing of beauty. Couchbase Mobile is still very βpreviewable,β but well developed.
You will have to handle potential update conflicts (CouchDB returns an HTTP status code of 409, so you can handle it accordingly - based on your business requirements).
My only complaints are the boot time of the CouchDB service and the size of the dependencies. If you can get around these problems, this may be a good solution for you.
More details here: http://www.couchbase.com/products-and-services/couchbase-mobile
Good luck
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