Remove ASP.NET Core dependency and update to Rebus v5

[Hawxy]

Fixes #14

Breaking changes:

• .NET Standard 2.0 is now the minimum requirement
• In ASP.NET Core: app.UseRebus() -> app.ApplicationServices.UseRebus()

Notes:

• IBus disposal is handled by the service container, so the IApplicationLifetime hook was unnecessary and has been removed
• ApplicationBuilderExtensions.cs has been removed to get rid of the ASP.NET Core dependency (thus the one-word breaking change above)
• Samples and dependencies are based on .NET Core 2.1 due to 2.0 being End-of-Life.
• I've fixed up the existing tests so they still work, but it seems like they were never completed to begin with.

---

Rebus is MIT-licensed. The code submitted in this pull request needs to carry the MIT license too. By leaving this text in, I hereby acknowledge that the code submitted in the pull request has the MIT license and can be merged with the Rebus codebase.

[CLAassistant]

All committers have signed the CLA.

[mookid8000]

Excellent work 👍

One thing though: The buildserver says it's good, but it turned out it's because it didn't run the tests (don't know why...)

When I run the tests I get this result:

I understand that it's because the container adapter does not get disposed as required, but my knowledge of the Microsoft Extensions DI container is not sufficient to understand how it's supposed to work.

How would one usually ensure that disposing the container would dispose something in it?

[Hawxy]

My bad, I forgot to re-run those tests before PR'ing. I'll take a look.

Edit: Interesting, I can get the RealContainerTests to pass but not the ContainerTests

[Hawxy]

See #18