Small addition to enable the append_to_response param in getMovie

Hey dude,

I'm using this API wrapper in my WP plugin -> https://github.com/interconnectit/MovieDBPress

I wanted to enable the append_to_response param in the getMovie method so I could just do a single request. Now you can pass an array or comma separated list of additoinal data to get eg:

```php
$tmdb = new TMDb( 'abcde' );
$movie = $tmdb->getMovie( 550, array( 'images', 'releases', 'casts', 'trailers' ) );
```

Hope you like the addition!