Start of SSH2 Agent Forwarding implementation

As I am rather keen on having this work I've written code to support a SSH2 Agent Forwarding implementation. This PR does enable this functionality but as I am not super familiar with PHP or the design of this library I'm expecting some feedback or comments. 

This PR will address this current OPEN issue / feature request. 

https://github.com/phpseclib/phpseclib/issues/505
> I think this should instead be a method allowForwarding() or something like that. And maybe also allow an option to disable it again?

I made a similar suggestion:

https://github.com/phpseclib/phpseclib/pull/518#issuecomment-65942075

My proposed API:

```php
<?php
include 'System/SSH/Agent.php';
include 'Net/SSH2.php';

$agent = new System_SSH_Agent(true);

$ssh = new Net_SSH2('www.domain.tld');
if (!$ssh->login('username', $agent)) {
    exit('Login Failed');
}

echo $ssh->exec('pwd');
$agent->startSSHForwarding($ssh);
echo $ssh->exec('ls -la');
$agent->stopSSHForwarding($ssh);
```

Maybe enableSSHForwarding() / disableSSHForwarding() would be better tho.