Significant interface improvements for Lwt, Async and Mirage

• Add an explicit ctx content to track every conduit's runtime state.
• Allow the source interface for a conduit to be set.
• Support a password callback for the SSL layer (#4).
• [lwt] Add stop parameters in main-loop of the server (#5).
• Add Conduit_mirage with Mirage functor suport.
• Add ocamldoc of most interfaces.
• Add a CONDUIT_DEBUG environment variable to the Unix backends for
  live debugging.
• Add a conn value to the callback to query more information about the
  current connection (#2).
• Expose the representation of Conduit_lwt_unix.flow in the external signature.
  This lets library users obtain the original Lwt_unix.file_descr when using
  Conduit libraries like Cohttp.