* Implement paginator construction relative to a path resolved from an object

This facilitates pagination URLs to be derived from parametrized URL routes. Consider, for example, the following path pattern: `@"users/:userId/articles"`

Assuming that the response can be paginated when the "page" query string parameter is supplied, the following will create a paginator for the articles of a given user:

    RKPaginator *articlePaginator = [objectManager paginatorForObject:user path:nil parameters:nil withRelativePathPattern:@"?page=:currentPage"];
    [articlePaginator setCompletionBlockWithSuccess:^(RKPaginator *paginator, NSArray *objects, NSUInteger page) {
        /* success block */
    } failure:^(RKPaginator *paginator, NSError *error) {
        /* failure block */
    }];
    [articlePaginator loadPage: 0];

I wonder if there is a simpler way to achieve the same using the paginator code in the base repo. Thanks!