Wrap http.Server with a new type to expose internal functionality

I need to have access to the underlying `http.Server.ConnState` for one of my applications, but since `graceful` was using it, it wasn't possible. This adds a new type, `graceful.Server`, that wraps an `http.Server` while allowing `ConnState` to be set. This also allows other methods and fields of `http.Server` to be accessed by application code, which could be useful.