Enable server-side encryption when uploading a file

Added the option to specify a server-side encryption algorithm to use when Amazon S3 creates an object.

```php
<?php
    require 'amazon-s3-php-class/S3.php';

    $fileName = "test-file.txt";
    $bucketName = "test-bucket";

    $s3 = new S3("ACCESS KEY", "SECRET KEY");
    $s3->putObject($s3->inputFile($fileName, false), $bucketName, $fileName, S3::ACL_PRIVATE, array(), array(), S3::STORAGE_CLASS_STANDARD, S3::SSE_AES256);
```

You just specify `S3::SSE_AES256` as the option for the new `$serverSideEncryption` parameter.

Then log into the S3 interface and check that `Details > Server Side Encryption` is set to AES-256.