Closes #3

This PR introduces the implementation of word:audio requests.

An example of creating a **word:audio** request is,

```php
$c = new Picnik\Client;
$c->setApiKey('foobar');

$audio = $c->wordAudio('alfalfa')->useCanonical(true)->limit(1)->get();

var_dump(count($audio));
// int(1)

echo $audio[0]->fileUrl;
// "http://api.wordnik.com/v4/audioFile.mp3/06e1ddb5a77443a1514960ca09fa90e2dfaf5b4242474a67bc3c9b5715eaae7d"
```

Note that the URLs that are created by the API are temporary and time-limited. Thus, the request should be made when the audio is needed. The audio content may also be cached, but that is dependent upon the agreed terms-of-service with Wordnik allowing you to do so.