Add a new complex sample using Nancy

https://github.com/AspNet-OpenIdConnect-Server/Owin.Security.OpenIdConnect.Server/issues/43

Note: this PR should not be merged before the following points are added:
* A correct documentation for `Nancy.Client` and `Nancy.Server`.
* A repository (probably using __EF6__) storing application details and their credentials.
* An updated `CustomOpenIdConnectServerProvider` using the repository.
* An updated `TestAuthenticationTokenProvider` storing __authorization codes__ in a permanent store (probably using the repository)