Turn obj into a proxy for target. This forwards both function calls and property setters/getters.
function forward(obj, target, onlyAccessors) {
  onlyAccessors = onlyAccessors || false;
  for (var k in target) {
    (function(k) {
      if (typeof(target[k]) == 'function') {
        if (!onlyAccessors) {
          obj[k] =  target[k].bind(target);
        }
      } else {
        Object.defineProperty(obj, k, {
          get: function() { return target[k]; },
          set: function(x) { target[k] = x; }
        });
      }
    })(k);
  }
}