Create a basic jsdoc externs file for Polymer

We should probably add `attributeChanged`, too:

```js
/**
 * Callback fired when an attribute on the element has been added, changed, or removed.
 *
 * @param {string} name The name of the attribute that changed.
 * @param {*} oldValue The previous value of the attribute, or null if it was added.
 * @param {*} newValue The new value of the attribute, or null if it was removed.
 * @param {string} namespace The namespace of the attribute.
 */
PolymerElement.prototype.attributeChanged = function(name, oldValue, newValue, namespace) {};
```