We hope to move to AppVeyor in the near future off of Buildbot + EC2. This adds
an `appveyor.yml` configuration file which is ready to run builds on the auto
branch. This is also accompanied with a few minor fixes to the build system and
such to accomodate AppVeyor.
The intention is that we're not switching over to AppVeyor entirely just yet,
but rather we'll watch the builds for a week or so. If everything checks out
then we'll start gating on AppVeyor instead of Buildbot!
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