Refactor

* Make `RCRTableViewRefreshController` directly a `UIRefreshControl`. Next I would like to rename the class and project to `RCRRefreshControl`.
* No need to retain the temporary `UITableViewController` (at least in my tests).
* Remove no longer needed `endRefreshing`.

Ideas for another pull request:

* Create a `UIViewController` category that adds a `refreshControl` making working with refresh controls and normal controllers exactly the same as with table controllers.