add curtain functionality and refactor examples

This PR adds the capability to display an optional background layer while loading. This is useful in the cases of showing a spinner above text or images. 

I changed the API since after adding the `showCurtain` option the API would have many optional parameters. The new API takes the `color`, `x`, `y`, and `showCurtain` parameters as a key of an options object. 

Example:

```javascript
new Spinner(element, {color: 'black', x: '50%', y: '50%', showCurtain: true});
```

I refactored the current examples and also added a new one for the curtain one.