Add exec support, fix for unlink

ssh2_exec() support for remote execution of SSH commands.  Usage:

```php

$exec = $session->getExec();
$exec->run('touch hello');

```

This would execute the `touch hello` command on the remote server.

In addition I found an error in the Ssh\Sftp::unlink() method, it was using the incorrect function, and would never work properly.