Accessible accordion

updated the template to valid html semantics

added accordionTranscludeId directive so that by setting the id on accordion-group, it sets the heading link to the content's id

@bc-edlou @SiTaggart @alexmckenley 
```
<dd>
  <a ng-click="isOpen = !isOpen" ng-class="{ active: isOpen }"  accordion-transclude="heading" ng-href="{{ '#accordion-' + id }}" accordion-transclude-id="id">{{heading}}</a>
  <div class="content" ng-style="isOpen ? {display: 'block'} : {}" ng-attr-id="{{ 'accordion-' + id }}" ng-transclude></div>
</dd>
```