Supervisor fixes

This pull request addresses the issues in #40

However - for the life of me I cannot solve a synchronization problem with the unit test I added in https://github.com/Aaronontheweb/Pigeon/blob/745ad9adec393882769b10a775c64eba49885ba0/test/Pigeon.Tests/Actor/ActorLifeCycleSpec.cs#L374

The final assertion fails, but it's only because there's a couple of mailbox round-trips that have to happen between the children and the supervisor (deathwatch notifications, etc...)

I can verify that this has solved my issue in production code, but fixing this test has been challenging. Any ideas?

Best,
-Aaron