allows container classes to update when multiple instances

we had a case where we had 2 Nerdbox instances on the same page that required their own styling.  Once the container was added to the DOM, the classes wouldn't get updated even when the new instance replaced the Nerdbox content as expected.  setting the the classes outside of the container seems to be the simplest way to achieve this.  
The original location was definitely not the right spot to add the classes, but I think this location suffers from similar problems. For instance, I suspect a scenario like this to fail because the class management is happening when the object is initialized:

```javascript
var nerdbox1 = new Nerdbox('.nerdbox-foo', {classes: 'foo'});
new Nerdbox('.nerdbox-bar', {classes: 'bar'});
nerdbox1.open(); //=> Would expect foo class, but has bar class because classes are added on init
```

Thanks so much for the PR. Can you restructure the spec to cover both cases and maybe move the `#addClass` behavior to Nerdbox's `#open` method, so the classes get added when each instance is opened instead of initialized?