Use Nancy self-host instead of IIS Express.

- Move most of the code to a new assembly, WelcomePage.Core;
  this allows easy testing via a web application, while still making it easy to use
  from the VS package.
Fix up VS extension.

- Remove SNK file; Nancy isn't strong-named.
- Tweak .NET versions.
- Remove ZIP file asset -- it's all included in the same process now.