I don't want to sound negative towards you project, but why not use CouchDB [0], along with TouchDB [1] for mobiles and PouchDB [2] for the browser ?
Edit: I see what you mean: Pouchdb-server [1] is a couchdb-compatible server that speaks the same sync protocol. If you don't want to install it, you can work with any hosted couchdb-speaking cloud instance, such as iriscouch [2] or cloudant [3]
[0] http://pouchdb.com/getting-started.html