Joining MUC without receiving discussion history

I'm working on a XMPP bot working mainly on MUC. When I was connecting to the server I got recent discussion from a room making bot to interpret old messages. I added `$options` (array) argument in `join_room` method of `XEP_0045` class. If you pass an array with `no_history` element set to `true`, `<history>` tag will be sent do server and you won't receive recent history. Example:

```
$client->xeps['0045']->join_room($room_full_jid, array('no_history' => true));
```

(http://xmpp.org/extensions/xep-0045.html#enter-managehistory)

I also changed `examples/muc_log_bot.php` to allow setting host in a script argument.