accessible accordion

@SiTaggart @alexmckenley 
Personally, when it comes to accessibility, I believe you should use browser native behaviour where you can. It saves on unnecessary event bindings even if the mark up looks a little less attractive. 

IMO get rid of the binding and add the href:
```
<a ng-click="isOpen = !isOpen" ng-class="{ active: isOpen }"  accordion-transclude="heading" href="#" tabindex="0">{{heading}}</a>
```

@bc-ong might have something to say on that too.