Add AppVeyor CI (Windows) support

[malept]

As of ruby-sys 0.2.3, ruru-based extensions can be built on Windows. It would be nice to add CI support for Windows builds.

A starting point can be found at my rusty_blank project: https://github.com/malept/rusty_blank/blob/master/.appveyor.yml

Some notes about my config file:

• You probably want to test against stable, beta, and nightly (I only needed stable there).
• I intentionally omitted MSVC builds from my AppVeyor config because the popular RubyInstaller for Windows is built with the MinGW toolchain.
• Ignore the artifact/deploy sections - that is for my project to distribute Rust+Ruby extensions.

[d-unsed]

Hi @malept!

Thank you, I'll take a look at it tomorrow

[d-unsed]

Thanks, AppVeyor support enabled!