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.
To explain in a little less brevity, I'm installing Julia to `C:\projects\julia`, so the julia executable lives at `C:\projects\julia\bin\julia`. AppVeyor clones your repository into a slightly regularized folder name, for this I think it ends up in `C:\projects\sqlite-jl` or something similar. That's why I end up needing the extra package name input to `Pkg.clone`, otherwise it doesn't recognize that `pwd()` corresponds to a package name. I'm not moving around with `cd` or anything, so the `test\runtests.jl` is a relative path w.r.t. where AppVeyor has cloned the repo.