AFNetworking/AFIncrementalStore — pull request #169 by dulaccc (2013-03-16 12:18:28+00:00)
Title: 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.