Create appveyor.yml, use unpacked_dir option in BinDeps

Due to overlapping status indicators, I also recommend turning off AppVeyor on pull requests for now, see https://github.com/JuliaLang/JSON.jl/pull/69#issuecomment-44780033

Build results should be visible here https://ci.appveyor.com/project/tkelman/sundials-jl/build/1.0.3, there's currently a test failure on Win32