Windows CI using AppVeyor

AppVeyor is a CI service much like Travis, but for Windows. It takes about 10 minutes to set up all the dependencies for Requests, but once it does the tests all pass on both 32 bit and 64 bit Windows (with some deprecation warnings): https://ci.appveyor.com/project/tkelman/requests-jl/build/1.0.3

Hopefully useful to get this running so it stays that way.