Separating Sass code into core, theme and config modules

I separated the Sass code into 3 separate files for easier look and feel updates. 

* `_modalConfig.scss`
* `_modalCore.scss`
* `_modalTheme.scss`

I'm using the plugin for our current project and the only downside was how the look and feel was tied to the core styles.

This has helped me since the theme styles are separated from the core and the risk of breaking the functionality is reduced.

The only actual change is removing the BG data image which I replaced with CSS styles to allow customization.

**Replaced:**

```scss
// Background for overlay – Data URI because of IE8 not supporting rgba
background: url('data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAEElEQVR42gEFAPr/AAAAAMwA0QDNTiUx4gAAAABJRU5ErkJggg==');
```

**With:**

```scss
background: #000;
background: rgba(0,0,0, $modal-bg-opacity / 100 );
filter: alpha(opacity=$modal-bg-opacity);
```

Feel free to ignore if this is not going in the direction you want!