Make flash an object, not an array.

This allows for iterating over all the flash object (in handlebars at least), and using the keys to set the class, rather than checking if we have a flash.error, flash.info, etc…
```
{{#each flash}}
  {{#each this}}
    <div class="alert alert-{{@key}}">
      <button type="button" class="close" data-dismiss="alert">&times;</button>
       {{this}}
    </div>
  {{/each}}
{{/each}}
```