Allow for any button type in toolbar by using .btn in selector

Changed bindToolbar selector for finding buttons from a to .btn when listening to click events.

This allows someone to use a `<button>` or an `<a>` tag in their toolbar. This change also makes the two toolbar button selectors in the code match as the updateToolbar method uses .btn and not a:

```javascript
// in updateToolbar
$(options.toolbarSelector).find('.btn[data-' + options.commandRole + ']').each(function () {
```