Exposed RKObjectRequestOperation within RKPaginator

Reopening as a new pull request for issue #1332

In order to determine a request's status inside of its completion blocks, RKObjectRequestOperation either need to be visible inside of RKPaginator or returned in some other fashion in the response. There were a few methods proposed to do this and I have implemented it one way here. 

RKObjectRequestOperation is now exposed within the RKPaginator instance. When receiving a response in the completion block, you can call [paginator objectRequestOperation] for the operation performed during the most recent call to RKPaginator. If you call -loadPage: or -loadNextPage the objectRequestOperation property would retain a new RKObjectRequestOperation for this latest action. This is in line with the other properties retained by the RKPaginator e.g. currentPage.

I have played with also changing the block implementation for success and completion to return the RKObjectRequestOperation but the change would be more impactful than exposing it in RKPaginator.