Add a strong name signing key and activate signing

[cptjazz]

In an effort to increase compatibility to projects using strong name signing and in an effort to eventually add strong name signing to Akka itself, this PR adds a signing key and activates signing.

Please also cf. https://docs.microsoft.com/en-us/dotnet/standard/library-guidance/strong-naming and https://www.pedrolamas.com/2018/09/11/start-strong-naming-your-assemblies/

[cptjazz]

This PR closes issue #80