AppVeyor build should build VS2013 samples with VS2013 VSSDK installed

[AlexEyler]

There are some samples coming (#119) that require the VS2013 VSSDK. Rather than installing it for all AppVeyor builds in this repo, we can install the VS2013 VSSDK for the subset of samples that require it.

More info on how to do it: https://www.appveyor.com/docs/build-environment/#using-multiple-images-for-the-same-build.