Enable passing html blocks into button

Useful if you want to include a spinner for the pending state.

Syntax:
```
{{#async-button action="save" class="html"}}
  {{#button-state name='default'}}
    <strong>Save</strong>
  {{/button-state}}
  {{#button-state name='pending'}}
    <strong>Saving...</strong>
  {{/button-state}}
  {{#button-state name='fulfilled'}}
    <strong>Saved!</strong>
  {{/button-state}}
  {{#button-state name='rejected'}}
    <strong>Fail!</strong>
  {{/button-state}}
{{/async-button}}
```
Don't use the Extended prototypes for external libraries. Please use the verbose style:

```js
active: Ember.computed('parentView.textState', function() {
  ...
})
```