Added 'setBtnClasses' function

Hi There, 

I needed a way to override the default CSS button classes for the prompt/alert/dialog functions, so I added the `bootbox.setBtnClasses` function. 

Example usage: 

```javascript
bootbox.setBtnClasses({
    CONFIRM: 'btn-danger',
    CANCEL: 'btn-primary',
    OK: 'btn-inverse'
});
```

I've also added a test case for this. Let me know if you spot any problems!