Enhancement/fix for #81

This will allow the class specified on a column to be applied to the ``` <th> ``` row even if that column is of type checkbox or radio.  The default is opt-in, so as to remain backwards-compatible.  The modifications will eliminate the errant behavior noted in the screenshot on the original issue report.

Example:
```javascript
            }, {
                field: 'COMPLETE',
                title: 'Complete',
                align: 'center',
                valign: 'middle',
                sortable: true,
                checkbox: true,
                checkboxEnabled: false,
                chkBoxStyled: true,
                class: 'info'
            }, {
```