Add CSS class to length select element

I was looking for a way to style the SELECT element from the length feature.
There is option to add a css class to the entire menu wrapper but not to the item itself. 
For bootstrap 3 styling the class "form-control" is required on this select element.

I added **sLengthMenu** to the classes which you can extend.

```
/* Default class modification */
    $.extend( $.fn.dataTableExt.oStdClasses, {
        "sLengthMenu": "form-control input-sm"
    } );
```