Create appveyor.yml

You like cross-platform testing, yeah? Who doesn't like cross-platform testing? See https://ci.appveyor.com/project/tkelman/sqlite-jl/build/1.0.2 for what this looks like. It's good for the sake of BinDeps and WinRPM to have more active packages running these regularly, so we can catch issues a little sooner - until PkgEvaluator runs cross-platform, anyway.

Signing up for an account on appveyor.com is pretty simple, if you haven't done it yet. After turning this on you'll also want to follow the instructions at http://github-multi-status.herokuapp.com to enable a webhook so Travis and AppVeyor can coexist on PR status indicators.
Is this really a thing? I would have guessed something like `C:\projects\julia\test\runtests.jl` or something like that.