feat(accordion): convert to bootstrap3 panel styling

This is effectively a template-only fix. I personally prefer the `<accordion>` markup, so I left that as is, and instead updated the template to reflect the changes to class names. This breaks with Bootstrap as they call theirs Panel Collapse, but Accordion makes more sense to me.

That said the templates are now somewhat confusing since `accordion.html` is now a simple `panel-group` and `accordion-group` is now a `panel`. To that extent the following markup would actually make more sense:

    <accordion-group close-others="oneAtATime">
        <accordion heading="Static Header">
            This content is straight in the template.
        </accordion>
        <accordion heading="{{group.title}}" ng-repeat="group in groups">
            {{group.content}}
        </accordion>
        <accordion>
            <accordion-heading>
                I can have markup, too! <i class="icon-check"></i>
            </accordion-heading>
                This is just some content to illustrate fancy headings.
        </accordion>
    </accordion-group>

Not sure that's worth breaking backwards compatibility, but it certainly makes more sense with how Bootstrap 3 does accordions.