Add Appveyor script for building Windows wheels

[dougthor42]

This adds Appveyor CI for building Windows wheels.

• PyPy is not built because Appveyor doesn't come with PyPy pre-installed and I didn't want to figure out how to add it.
• A stub for releasing to GitHub is included but commented out, as it will require the repo maintainer to generate a token and then secure it in Appveyor.
• Wheel artifacts are made for every build and uploaded to Appveyor. It's out of scope of this PR to auto-upload them to PyPI.
• Addresses wheels for markupsafe #45, though I'm not sure if this PR actually closes it.

[dougthor42]

@davidism Please review. Having windows wheels will make many people's lives a lot easier.

[davidism]

I'm currently in the middle of going through Flask's backlog before PyCon, and don't have a lot of free time besides that. I'm aware of the PR, you don't need to keep pinging me.

[dougthor42]

Sorry about the annoyances! I'm used to contributing to projects that are barely maintained. I'll stop 😄