Add AppVeyor config file

[frankshearar]

Then we can have tests running on a Windows machine, as well as Travis' Linux machines.

[krono]

I'm in favour of appveyor,
however, I'd like to refactor the whole process here…
Stay tuned.

[frankshearar]

I'm happy for us to rip everything apart, but hooking up to AppVeyor is pretty orthogonal - it'll just call the Rakefile, just like Travis.

[krono]

Ok, go for it :)