Provided updated method

With this pull request the plugin receives an update method that allows updating the combobox element with jQuery, see #75.

```javascript
var val = 'my_value';
jQuery('#my_element').combobox('update', val);
```

There is still some room for improvement, for instance when the value of the select element has changed one could call the update method without any parameter and the text input changes to whatever is currently selected.