return promise proxy from Collection#fetch

This is useful to display loading messages, so in a route you can do:

```javascript
setupController: function(controller, model) {
  this._super(controller, model);
  controller.set(“messages”, this.store.fetch(“message”));
}
```

and in the template you can then do:

```handlebars
{{#if messages.isPending}}
  Loading messages...
{{else}}
  Loaded {{messages.length}} messages
{{/if}}
```
bump to v0.2.4