Feature/embedded belongs to support

adds support for saving embedded belongsTo relations

![screenshot](http://d.19degrees.io/hWI2+)

Model: Tag
```
export default DS.Model.extend({
	name: DS.belongsTo('translatable', {embedded: true}),
	slug: DS.attr('string'),
	products: DS.hasMany('product'),
	banner: DS.belongsTo('image'),
	avatar: DS.belongsTo('image'),
	hidden: DS.attr('boolean', {defaultValue: false})

});
```

Model: Translatable
```
export default DS.Model.extend({
	en: DS.attr('string'),
	zh: DS.attr('string')
});
````

Creation & Saving
```
var nameTranslatable = this.store.createRecord('translatable');
    	
var tag = this.store.createRecord('tag');

tag.set('name', nameTranslatable);

....

Ember.debug('saving new tag...');
tag.save();

````