Added pagination to subscribed subreddits.

I added another method to `RKClients+Users` for pagination on subscribed subreddits:

`- (NSURLSessionDataTask *)subscribedSubredditsInCategory:(RKSubscribedSubredditCategory)category pagination:(RKPagination *)pagination completion:(RKListingCompletionBlock)completion`

I don't know how you currently feel about breaking backwards compatibility, but for this to work I changed the old methods completion blocks to be `RKListingCompletionBlock` instead of `RKArrayCompletionBlock`. Other than that, I just had them filter in to the new method, which only has additions for an `RKPagination` object. Let me know how you feel about these changes.