Typos fix

Summary: Typos fix

Test Plan: Read through the changes
Added thumbnail_url, thumbnail_width, thumbnail_height to AdCreative

Summary: Added new fields pertaining to thumbnails in AdCreative

Test Plan:
```
$creative = new AdCreative('<AD_CREATIVE_ID>', 'act_11124366');
$resp = $creative->remote_read(array(
  AdCreativeFields::THUMBNAIL_URL,
));
var_dump($resp);
```