Dedicated private messaging sound

As per issue #52 I've implemented the changes necessary to support a different sound file for private messages, now that I've worked out how to do a pull request.

Included are language translations (aided by Google Translate) for all of the AJAX Chat language files except for 6 of them. Where a translation hasn't been provided, the contents of the ``` $lang['settingsSoundPrivate'] ``` variable has been left in English. The six untranslated additions are within:
* ja.php
* ka.php
* kr.php
* th.php
* zh-tw.php; and
* zh.php.

This AJAX Chat addition goes as far as setting up the ground work for a dedicated private message sound. All that is required past this is to change the line in config.js for the sound_7 mp3 filename to one of your choice. The default private messaging sound is sound_1.mp3, the same audio clip used for public messages received.

Here's what I mean:

```js
   // Defines the available sounds loaded on chat start:
   soundFiles: {
     sound_1: 'sound_1.mp3',
     sound_2: 'sound_2.mp3',
     sound_3: 'sound_3.mp3',
     sound_4: 'sound_4.mp3',
     sound_5: 'sound_5.mp3',
     sound_6: 'sound_6.mp3',
     sound_7: 'sound_1.mp3'
   },
```

I see the author of the 6 original AJAX Chat sounds (http://www.soundsnap.com/user/21) has many others available, so perhaps one additional sound file of the same style could be sourced from there and included with AJAX Chat releases as sound_7.mp3? Example: http://www.soundsnap.com/node/8460