Updated ToggleButton.options.content to work with multiple states

Regarding #354

This update will not break existing implementations

Changes this:
```javascript
var button = new ToggleButton({
    content: 'on'
});
button.offSurface.setContent('off');
```
To this:
```javascript
var button = new ToggleButton({
    content: ['on', 'off']
});
```