Fixed activity to use font awesome spinner.

* Dropped jquery dependency.
* Added AMD wrapper.
* Support custom active/inactive classes (for use without font awesome).

This version is much shorter than the original. It also works without font awesome, by allowing users to specify a custom element and class combination to use for signaling active and inactive states.

For instance:

```html
<button class="btn btn-large" data-bind="active: save.isActive, activeOptions: {container: 'span', activityClass: 'busy', inactiveClass: ''}"> Save</button>
```

This will insert a span container inside the button and apply class 'busy' when the button is active, and no classes when the button is not active.