Make Spool.Empty public object

In the following, for example, we should implement `case _ =>` pattern to prevent to get `MatchError` when `update` is invoked with `Future.value(Spool.empty[Int]))`:

```
import com.twitter.concurrent.Spool
import com.twitter.concurrent.Spool._
import com.twitter.util.Future

def update(changes: Future[Spool[Int]]) {
  case i *:: tail => println(i); update(tail)
  case _ => // Empty
}
```

If `Spool.Empty` is public, we can clearly state our intention like the below:

```
def update(changes: Future[Spool[Int]]) {
  case i *:: tail => println(i); update(tail)
  case Empty =>
}
```