Router specs + more

Changes:

* Added `Group.Props()` to group routers

This makes it possible to do:

````csharp
var routedActor = system.ActorOf(new RoundRobinGroup(paths).Props());
```

* Extracted SmallestMailboxSpec.cs from RouterSpec.cs
* Added BroadcastSpec
* Added RandomSpec
* Added RoundRobinSpec
* Added ScatterGatherFirstCompletedSpec
* Some refactoring in `SupervisorStrategy` and its subtypes
* Removed conditional compilation for build server failing tests, just using `Skip`

Some of the specs are incomplete..