Add an experimental Appveyor Windows/Cygwin32 build file

This uses the Appveyor CI service (similar to Travis) to build
OPAM on Cygwin on Windows.  It currently installs fine, but then
runs into ocsigen/lwt#84 when installing Lwt.

See ocaml/opam#246
Merge pull request #1 from avsm/appveyor

Experimental Windows CI build
Merge pull request #1764 from avsm/master

Experimental Windows CI build using Appveyor