added "key" authentication method

To authenticate without user interaction we need to use a "server key" (see attached image) provided by the google cloud console.
This is useful if you want to access public data that is not associated with an account.



To use it just call:

```javascript
Youtube.authenticate({
    type: 'key',
    key: 'AIzaSyAN2S1NSmmUEWGa1ev9fFyIpWsBPNogOtE'
});
```

![google cloud console](https://f.cloud.github.com/assets/1411972/1712872/4b605cc8-617f-11e3-9852-1372037f8ab0.png)