Added function to allow multi-dimensional arrays in params

In case anyone is still using this useful wrapper I added the ability to have multi-dimensional arrays in params

Example Command: getTransactionList

```PHP
$adminapi = new SagePayAdminApi('vendor', 'username', 'password');
$transaction = $adminapi->getTransactionList(
                    array(
                        'startdate' => '01/01/2013 00:00:00',
                        'enddate' => '02/01/2013 00:00:00',
                        'txtypes'=>
                            array(
                                'key'=>'txtype',
                                'AUTHORISE',
                                'AUTHENTICATE'
                            )
                        )
                    );
```