Expose main lwt promise

[shawn-mcginty]

Naboris.listen encapsulates the Lwt_main.runing of the promise. There should be a function to get the raw promise allowing the user to manage his own top level Lwt_main.run

Something like Naboris.listen_promise: int serverConfig -> 'a Lwt

[shawn-mcginty]

in 0.0.7 Naboris.listen was renamed to Naboris.listenAndWaitForever and there is a new Naboris.listen which returns an lwt promise