I added support for passing a function as input to the selectClass option

So you can pass a function into the selectClass option. Pass $el as 'this' to the function to allow context-aware class definitions.

So you can do something like this:

```javascript
$('select').uniform({
  selectClass: function() {
    var cls = this.data('uniform-class');
    return (cls !== undefined && cls.length > 0 ? 'selector ' + cls : 'selector');
  }
});
```

What do you think?