Add parameter for method 'getStats'

[Timandes]

Added one optional parameter args 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);

[sodabrew]

I like this! Could I ask you to rebase to master branch (the php7 branch is merged to master now) and to also add the optional parameter to the memcached-api.php doc file?

[Timandes]

See #298