Windows dev builds now available via Appveyor

Hey all,

In case you weren't following on Github or aren't on sc-users, we're now
using Appveyor to provide continuous integration support for Windows. This
is a pretty nice deal; it's already helped us catch a few issues related to
the differing C++ feature support and diagnostics offered by MSVC.
Previously, these would have been detected manually, possibly at a later
stage where it would be harder to address.

The completed builds are also zipped and uploaded as artifacts to Amazon
S3. You can download builds of the latest develop branch build at
https://supercollider.github.io/, and change the appropriate part of those
links' URLs to get builds on other branches (not master, because the config
file hasn't been merged there yet).

Also, exe installers will be automatically uploaded to GitHub releases,
just like Mac builds are.

Currently, no tests are run on Windows builds on Appveyor. However, I
recently did some work to get Scott Carver's qpm working on Windows, which
means that it's within the realm of possibility that we could soon have the
actual testsuite from the main repository running on all systems during CI.

If anyone wants to help with that, PLEASE let me know. It's one of my top
priorities but I only have so much time because, grad school. I'd really
appreciate the help.

Regards,
Brian