implemented album cover retrieving, now with more nodejs style api

as discussed before in #24, heres an optimized version of the implemenation to fetch cover images for an album. the callback has now a nodejs style signature:

```javascript
album.smallCoverImage(function(error, buffer) {
    if(!error) {
        console.log('we have an image!');
    } else {
        console.log('nay :(', error);
    }
});
```

glad if you decide to merge :)