appveyor: Initial integration.

appveyor: Initial integration.

AppVeyor doesn't require an appveyor.yml in the repos (in fact it has
some limitations as noted in comments below), but doing so has two great
advantages over the web UI:

- appveyor.yml can be revisioned together with the code, so instructions
  should always be in synch with the code

- appveyor.yml can be reused for people's private repositories (be on
  fdo or GitHub, etc.)

Acked-by: Roland Scheidegger <sroland@vmware.com>