Add optional parameter 'type' for method 'getStats'

[Timandes]

Added one optional parameter type for method 'getStats'.
Example:

<?php
$m = new Memcached();
$m->addServer('localhost', 11211);
$r = $m->getStats();
var_dump($r);
$r = $m->getStats('settings');
var_dump($r);

( Old PR #264 )