function extend_(destination, source) {
  var keys = Object.keys(source);
  for (var i = 0; i < keys.length; ++i) {
    destination[keys[i]] = source[keys[i]];
  }
  return destination;
}
Copy all of the properties in the source objects over to the
destination object, and return the destination object.
@param {Object} destination The combined object.
@param {Object} source The object who's properties are copied to the
    destination.
@return {Object} A combined object with the desination and source
    properties.
@see http://underscorejs.org/#extend