Fixed "Class 'S3\Exception' not found".

Fixed "Class 'S3\Exception' not found".

▼Error

ErrorException [ Fatal Error ]:
Class 'S3\Exception' not found

PKGPATH/fuel-s3/classes/s3.php @ line 290

▼ Before

```php
throw new Exception('S3::input_file(): Unable to open input file: ' . $file);
```

▼ after

```php
throw new \Exception('S3::input_file(): Unable to open input file: ' . $file);
```