Add ClusterSharding steps

Hi Heiko,
I found this project (and `sbt-groll` – really awesome job) very useful for demoing persistence, kudos :-)

I've added a simplistic `ClusterSharding` example, as people ask about it after showing the Singleton anyway, and we'd rather have people use Sharding than Singletons to solve their problems :-)

Two commits:
* adds cluster sharding
* adds passivation
Sure, I forgot that there are multiple user-service actors now.