Support belongs to

(I sent new PR because I couldn't stack up to #96. I'm sorry.)

I want to use 'references' in scaffold generator.

When the following command run, I hope that the model was generated validly.
`$ rails generate scaffold comment post:reference body:text`

expected:
``` js
App.Comment = DS.Model.extend({
  post: DS.belongsTo('App.Post'),
  body: DS.attr('string')
});
```
But actual:
``` js
App.Comment = DS.Model.extend({
  post: DS.attr('references'),
  body: DS.attr('string')
});
```