Add support for foreign relationship identifier

A lot of APIs are now referencing relationships with an hyperlinked identifier or an id. 

Mine for example looks like this :  
```ruby
{
    "url": "http://localhost:8000/api/coupons/39",     # object identifier
    "user": "http://localhost:8000/api/users/1",       # relationship identifier
    "date_created": "2013-03-16T11:36:11.687345+00:00", 
    ...
}
```

I think if the identifier is an ID, the developer will also need to implement `requestWithMethod:pathForRelationship:forObjectWithID:withContext:` to use the correct api request path. I don't think we should take a decision for him here, that's why I didn't support that.

This is clearly linked to the PR #62, nevertheless I thought it'd be clearer with only those few lines.