Windows CI using AppVeyor

If you guys want to turn this on. There are actually some Windows test failures in master right now, slightly different between 32 bit https://ci.appveyor.com/project/tkelman/json-jl/build/1.0.7 and 64 bit https://ci.appveyor.com/project/tkelman/json-jl/build/1.0.8/job/bn9rkpoojoq6ciqb.