Remove TestKit's AtomicInteger and replace with Akka.Utils.AtomicCounter
Move Comparable from Testkit to Akka.Tests, to remove dep on fastJSON
Remove HoconTests from TestKit and duplicate in Akka.Tests
Move ImplicitSenderSpec to Akka.Testkit.Tests
Move Supervisor to Akka.Tests
Change namespace for AkkaSpec & TestProbe to Akka.TestKit
Implement TestKit.TestActor

Also fix tests since the AkkaSpec queue now contains MessageEnvelopes.
Move types in AkkaSpec.cs to matching files
Extract parts of ActorSystem into ActorSystemImpl, ExtendedActorSystem. Breaking Changes!

ActorSystem is the public interface, and it has been made abstract. To create a system use ActorSystem.Create

ExtendedActorSystem is what extensions sees, and ActorSystemImpl is used internally.
Implement TestKitBase and Xunit.TestKit. Breaking changes

- Most of TestKitBase is implemented
- Added a TestKit for xUnit.
- Moved AkkaSpec out of TestKit, It's for internal testing and is not part of TestKit
- ActorSystem contains a AwaitTermination that can be used for waiting for system shutdown
- Added operator + on Config, so you can concat a string with a Config. For example: config + "akka.loglevel = DEBUG" and the other way around.
- Other changes are due to breaking changes in TestKit (Making public members PascalCase, and renaming)
Merge pull request #214 from HCanber/testkit-182

Restructure and refactor TestKit.
See https://github.com/akkadotnet/akka.net/pull/214 for more info.