Improved SNI support

This is a backwards compatible change that detaches the URL router from the App, and moves it underneath the SNI support (if any). The new hierarchy is: App -> (SNI) -> URL router.

One App can have the root level SSL certificate and its URL routes, and/or many per-domain SSL certificates and their own isolated URL routes.

You "browse" between different SNI domains by calling the new function app.domain(server name) before calling app.get or app.post or any such route registration call. This can be chained together in builder pattern. Last "browsed to" SNI is under where you register the URL route handler.

See example ServerName for usage.