ClaimsPrincipal support

This is a work-in-progress PR to get early feedback. 

As ClaimsPrincipal is .net4.5 only, we need a .net4.5 dll that provided this support. Since this support is based on the owin "server.User" env key, I've added a Nancy.Owin45.csproj. 

Also removed the obsolete Nancy.Hosting.Owin projects.