Return a slider reference w/ control API.

I started using this widget in a backbone app and needed the ability to control the slide from other views that are managing the state of the app. Didn't change any of the functionality, but did clean up the state handling and expose an external API.

EX:
```javascript
var menu = $('.menu-link').bigSlide();
menu.open();                 // open menu
menu.close();              // close menu
menu.open();                // open menu
$('.menu-link').click();    // trigger click, closes the menu since it's open

```