Improve connection closing semantics

When terminating conduits, always close the output channel first before the input channel, so that any pending data in the underlying fd is flushed.

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.

Enable Async SSL support

This release reenables Async SSL by default, since the upstream bug that prevented linking from working reliably has been fixed.

First public release

Async SSL is temporarily disabled in this release until a stub-generation version of the underlying library binding is available.