Move System_SSH_Agent for psr-0

hi.

System_SSH_Agent does not conform to the psr-0. And so can not load by autoload.
underscore of class name should be directory separator.

```console
$ php composer.phar require "phpseclib/phpseclib:*"
$ vim sample.php
```

*sample.php*

```php
<?php
require_once __DIR__ . '/vendor/autoload.php';

$agent = new System_SSH_Agent();
$ssh = new Net_SSH2('example.net');

if (!$ssh->login('your', $agent))
{
    exit('Login Failed');
}

echo $ssh->exec('uname -a');
```

```console
$ php sample.php
Fatal error: Class 'System_SSH_Agent' not found in ...
```