Hooking the sync

TODO: document
function hookSync(rs) {
  if (rs._dropboxOrigSync) { return; } // already hooked
  rs._dropboxOrigSync = rs.sync.sync.bind(rs.sync);
  rs.sync.sync = function () {
    return this.dropbox.fetchDelta.apply(this.dropbox, arguments).
      then(rs._dropboxOrigSync, function (err) {
        rs._emit('error', new Sync.SyncError(err));
        rs._emit('sync-done');
      });
  }.bind(rs);
}