Support for changing icons by _setOption #2

1st Change: 
 - Support for jQuery UI 1.8.x
- Tested with jQuery UI 1.8.23 + jQuery 1.8.2

2nd Change: 
 - Support for changing icons by _setOption.
- Tested with jQuery UI 1.8.23 + jQuery 1.8.2

e.g:
````
$("#MySelectmenu").selectmenu("option","icons",[
 {
  icon: "ui-icon-alert",
  find: ".MySelectmenu_Item_0"
 }])
````

Best Regards,
Alexander Bulei