Added custom curl opts.

Added custom curl opts that will be used in all subsequent requests.
Will not overwrite default or specific curl options!

Usage:
```php
$vimeo = new \Vimeo\Vimeo($client_id, $client_secret, $access_token);
$vimeo->setCustomCurlOpts(array(CURLOPT_SSL_VERIFYPEER => false));
```

Closes #12.
This can probably be dropped. I don't think we need public access, and we generally don't add public functions until they are explicitly needed.