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)