Move NancyOwinHost to core.

I think it's time we address this and start a discussion on how to structure/package things moving forward, and what better way to do that than with a pull request? :wink:

This PR renames`NancyOwinHost` to `NancyMiddleware` and moves it to the core project, (still) under the `Nancy.Owin` namespace.
The `AppBuilderExtensions` are left intact in the `Nancy.Owin` project. API-wise, this is not even a breaking change :+1:

I also added extensions for the `Action<Func<AppFunc, AppFunc>>` approach for adding middleware, used in the new [`IBuilder.UseOwin`](https://github.com/aspnet/HttpAbstractions/blob/dev/src/Microsoft.AspNet.Owin/OwinExtensions.cs) extensions. This means that you can now do

```csharp
public void Configure(IBuilder app)
{
    app.UseOwin(owin => owin.UseNancy());
}
```
in ASP.NET vNext, without other dependencies than core Nancy :dancers:

Fixes #1387