Render icons in a span tag when using a class name

More information about why this change: https://github.com/sproutcore/sproutcore/issues/963

This pull request include:
- replacement of icons `img` tags by `span` tags when using a class name
- update of the CSS rules to match that change
- wrapping of the CSS rules of the radio view into `sc-radio-view`
- icon `margin-right` fix for the button views
- icon positioning fix of 18px button views
- rename of the label classname of the checkbox view from `label` to `sc-button-label` for consistency.