Add ReceiveActor and MatchBuilder

Also updated PingPong to perform tests against ReceiveActor
Merge pull request #225 from HCanber/receive-builder-104

Add ReceiveActor and MatchBuilder
Created ReceiveActor and MatchBuilder.
Also updated PingPong to perform tests with ReceiveActor

Apart from changes in PingPong, nothing has changed.
Doc will come, but in the meantime, look at src/core/Akka/Util/MatchHandler/README.md
Replace usage of AtomicInteger with Akka.Utils.AtomicCounter
Move Comparable to Akka.Tests in order to remove dep on fastJSON
Remove HoconTests from Testkit

We have the tests already in Akka.Tests but it's missing CanUseFallback and CanUseFallbackInSubConfig
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!

This means that ActorSystem now only exposes public things.
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
- Other changes are due to breaking changes in TestKit (Making public members PascalCase, and renaming)