Sync classes

Keeps the `$control` classes synced up with the `$input`.

This is especially useful when using the Bootstrap 3 theme.

With this change, users can apply styles to the input on `.ng-dirty.ng-invalid` like this, for example:

```css
input.ng-dirty.ng-invalid,
textarea.ng-dirty.ng-invalid,
select.ng-dirty.ng-invalid,
.selectize-input.ng-dirty.ng-invalid {
  border-color: @state-danger-bg !important;
}
```