Expose dataType on ajax model

Was playing around with TBone this evening and I noticed that I was getting a return type of a `string` when referencing an `ajax` model set on the http://httpbin.org/get url:

```javascript
T('tracks.get', tbone.models.ajax.make({
    url: function() {
        return 'http://httpbin.org/get';
    }
}));
```

This was a bit surprising, seeing as the document type is `application/json`. So after digging around the code, I noticed that the `dataType` is hardcoded to `text` on the `ajax` model:

https://github.com/appneta/tbone/blob/87783b341ececcc162e1622ea05320ff20dad0e3/src/model/fancy/ajax.js#L31

Patch incoming!