add/remove arrays of classes

See [ampersand-dom-bindings#24](https://github.com/AmpersandJS/ampersand-dom-bindings/issues/24)
this is very nitpicky, but please keep the `else if` on the same line

```js
} else if (el.classList) {
```

Other than that, i think this is ready to go. Thanks for the very detailed issue on the other repo.