Add Appveyor control files

Note that these will not activate appveyor testing, as the pypa/pip project has not been registered with appveyor. However, it will allow forks (such as mine) to add appveyor testing without needing to maintain a separate branch to do so.

Ultimately it would be good to activate appveyor on the main repository, but I would rather wait until the tests pass, and we are sure that we can do so without interfering with the Travis builds.