Fixes #746 ability to now style checkboxes with the utility colors

Fixes #746 with the ability to now style checkboxes with the utility colours. No specified color class defaults to stable style.
Checkboxes can now be styled like the following: 
```
           <li class="item item-checkbox">
                <label class="checkbox checkbox-positive">
                    <input type="checkbox">
                </label>
                Flux Capacitor
            </li>
```