Add ServiceCollectionExtensions and ServiceProviderMiddlewareResolver

[mariusz96]

I could not stand that this is newing things up in the DI-enabled app so I went ahead and added:

• the factories to hide the newing (this is the best pattern I could find)
• the auto-registration methods for the IServiceCollection
• the IServiceProvider implementation of the IMiddlewareResolver
• the <GenerateDocumentationFile> property to the csprojs to include the documentation in the nuget and I fixed missing xml comment warnings

Let me know how you feel about the changes, I think they're a significant improvement over the previous version of PipelineNet.ServiceProvider.