Implement API to set MenuButton icon

Implemented an API method which allows setting the icon of MenuButtons. If icon is already present it is changed, if icon is not present it is created, if icon is present and ```null``` is provided it is removed.

```js
// MenuButton provided by any plugin
var menuButton;

// Set icon (replace if icon already set, add if no icon is currently set)
menuButton.icon("alignleft");

// Unset icon and remove icon node if currently set
menuButton.icon(null);
```