Windows CI using AppVeyor

AppVeyor is a CI service very much like Travis, but for Windows. DataFrames has been working nicely in Windows for me, but it's good to test that it stays that way. Oddly the `data.jl` and `io.jl` tests don't run properly on AppVeyor, I'm not sure why. The `io.jl` test fails locally for me when I do `Pkg.checkout("DataFrames"); Pkg.test("DataFrames")` so in that case it's not specific to AppVeyor. See test results for 32 bit: https://ci.appveyor.com/project/tkelman/dataframes-jl/build/1.0.1 and 64 bit: https://ci.appveyor.com/project/tkelman/dataframes-jl/build/1.0.2/job/1j3h47gf82f6n5dw