Further RACQueueScheduler changes

@joshaber

 * Made the initializer for `RACQueueScheduler` protected, because a caller using it directly could pass in a concurrent GCD queue, which would break the serialization contract of `RACScheduler`. Subclasses should expose an initializer themselves.
 * Re-added the `name` parameter to `RACQueueScheduler` so the name for `-description` is properly set.
 * Improved the default names used when `nil` is given.