Added alert component

Template was proposed here #22

Usage:
```html
<alert type="success">
  ...
  <a href="#" class="alert-link">...</a>
  <dismiss>&times;</dismiss>
</alert>
```
or
```html
<alert type="danger" dismiss="&times;">{{_session.flash.error}}</alert>
```
or initially hidden
```html
<alert as="testAlert" type="danger" hidden>
  {{_session.flash.error}}
  <dismiss>&times;</dismiss>
</alert>

<button on-click="testAlert.show()">Show alert</button>
```