Allow new CAS connection with same instance

In some case, we need to test more than on CAS server

Looks near to https://github.com/Jasig/phpCAS/issues/68

```php
/* Check different CAS */   
    for ($i = 0; $i < $nb_server; $i++) {     
        //Init connection
        phpCAS::client(CAS_VERSION_2_0,$config[$i]['host'],$config[$i]['port'],$config[$i]['uri'],true,true);
        phpCAS::setNoCasServerValidation();
        //Check authentification
        if ($auth = phpCAS::checkAuthentication()) {
            echo "server $i is ok"   
            break;
        }
    }
```