Add support for selecting the libev backend (more gently)

[aantron]

This is a version of #269 that warns current users of Lwt_engine.libev that the signature will change, as per #293.

• The new version is available as Lwt_engine.Versioned.libev_2. It will be released in the next Lwt release, but will replace the current libev in a major release.

• The current libev has a warning attached to it in the meantime:

  Warning 3: deprecated: Lwt_engine.libev
  This class will soon have parameters for selecting a libev backend. This will
  be a breaking change. See
    https://github.com/ocsigen/lwt/pull/269
  To preserve the current signature, use Lwt_engine.Versioned.libev_1
  To use the replacement immediately, use Lwt_engine.Versioned.libev_2 ()
  

Resolves #269

cc @yallop I modified the first commit slightly:

• renamed libev to libev' in the .ml file,
• defined libev in terms of that, and
• undid the change to the signature of libev

Hope that is ok.

[yallop]

Yes, that's fine. Thanks for taking care with git history & authorship!