dropdowns: use transclusion to improve the directive

Give more granularity and a more natural syntax to the <dropdown> directive by using transclusion and a new directive: dropdown-item

Example:
```html
<dropdown class="dropdown-menu-right" as-id="'myDropdown'">
    <dropdown-item as-href="'http://google.com'">Google</dropdown-item>
    <dropdown-item as-href="'http://twitter.com'">Twitter</dropdown-item>
</dropdown>
```