_extend = function() {
    var i, len, arg, prop, src, copy, args = _args(arguments), target = args[0] || {};
    for (i = 1, len = args.length; i < len; i++) {
      if ((arg = args[i]) != null) {
        for (prop in arg) {
          if (_hasOwn.call(arg, prop)) {
            src = target[prop];
            copy = arg[prop];
            if (target !== copy && copy !== undefined) {
              target[prop] = copy;
            }
          }
        }
      }
    }
    return target;
  }
Shallow-copy the owned, enumerable properties of one object over to another, similar to jQuery's `$.extend`.

@returns The target object, augmented
@private